Instead of using funtion `error!` the function `not_found!` is used to return 404 error. Adjusted documentation accordingly.

The API version is currently not equal to the gitlab major version number. Gitlab 4.1 
still uses API version 3. Point to the lib/api.rb file instead which contains the
autoritative information.

Also added tests.

The 2 reasons are :
- creation of user fail if name is empty : in after_save, it tries to create a
namespace with namespace.name = user.name and namespece validates presence
Namespace#name
- in the web app links to team members are broken with empty User#name because
they are of the form <a href'...'> user.name </a>

Added documentation for getting project hook and editing project hook.

[ci-skip]