Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

The API documentation of merge requests contains info to status codes for all functions.
Required arguments are now checked in the merge requests API functions and a `400 Bad Request` error is
returned if they are not given.

closes #2300

The issues documentation is updated with infos to status codes and the deprecated `DELETE` function and
how to close an issue. A few more tests added to check status codes of API functions.

Updates the API documentation of groups with infos to return codes. The function calls
in the groups API have updated documentation and return `400 Bad Request` status code
if a required attribute is missing.

The API documentation of repository is updated and now contains infos to status codes.
Code documentation is also adjusted for `GET /projects/:id/repository/commits` and includes infos to
pagination attributes. Tests are updated.

add grape-entity, that is split from grape 0.3.0.
remove usage of deprecated error_format, that is removed in grape 0.2.3.

Updated the milestones API documentation and added return codes descriptions.

The notes API documentation updated with return codes. API now returns `400 Bad Request` if
required attributes are not present. Return codes are documented now, also tested in added tests.
The documentation now reflects the current state of the API.

The users API updated with return codes, e.g. if required parameters are missing
a `400 Bad Request` error is returned instead of `404`. Fixes return codes of functions,
e.g. deletion of a ssh key is an idempotent function now.
The API documentation is updated to reflect the current status of the API. Descriptions
are more detailed and complete, infos to return values are added to all functions.

When a user is not authorized to see the list of hooks for a project, he is
still able to access the hooks separately. For example if access to
`GET /projects/:id/hooks` fails and returns a `403 Unauthorized` error it is
still possible to access a hook directly via `GET /projects/:id/hooks/:hook_id`.

Fixes access, also added tests to check access and status codes of hooks.

Remove hardcoded refernce to gitlab-shell home. so that gitlab can be installed on any unix account other than git

Status code 422 (Unprocessable Entity) returned if invalid url is given when creating
or updating a project hook.

When the project limit is reached the user is not allowed to create new ones.
Instead of error code 404 the status code 403 (Forbidden) is returned with error
message via API.

The previous call `saved?` is restored in the `POST /projects` method in the API.
It is refactored to check if the record is persisted. This is useful to not validate
the record again after saving. This fixes the returned status code in the web client
too. If the last project is created via web client instead of error notification
the project page is shown.

When creating the last project via API when reaching the project limit a status code
of 404 (Not found) is returned instead of 201 (Created). The fix checks now correctly if
the project could be saved.

Extracted a method for 400 error (Bad request) and adjusted code accordingly. The name of
the missing attribute is used to show which one was missing from the request. It is used to
give an appropriate message in the json response.

When using project snippets via API the functions now provide status codes for
different situations other then only returning 404 error. If required parameters are missing,
e.g. `title` when creating a project snippet a 400 (Bad request) error is returned. The snippet
delete function now is idempotent and returns a 200 (Ok) regardless if the snippet with the
given id is available or not. Changing return codes of these functions has the advantage that
the 404 error is used only for resources, which are not available.

Tests added to check these status codes when handling project snippets.

Fixing issue #2970

Different status codes in the API lib are returned on hook creation, update or deletion.
If a required parameter is not given (e.g. `url` in `/projects/:id/hooks/:hook_id`) status
code 400 (Bad request) is returned. On hook deletion a 200 status code is returned, regardless if
the hook is present or not. This makes the DELETE function an idempotent operation. Appropriate tests
are added to check these status codes.

Mac OS uses launchd instead of /etc/init.d to start daemons and tasks to be started by launchd MUST NOT daemon itself. So "nohup" here won't work for Mac OS. 

Can we add a "launchd" task to the rake file so that we can start sidekiq as "bundle exec rake sidekiq:launchd" ?