Fixes #1549

Document sync command in help

CONTRIBUTING: note license.

This is to be more explicit about the contribution process and license.

The default colored output isn't very useful when labels stand on their own.

Forgot to change expected result after changing labels functionality.

Add `hub pr list` command to list pull requests

Merging is for consistency of organization, since 'labels' is a
sub-command of 'issue', as is 'create', which is also inside
commands/issue.go.

Extract getSuitableTextColor in commands/issue.go as a result of
merger.

(based off commands/issue.go)

'hub issue labels' makes more semantic sense to retrieve all
available labels from the issue tracker. The command 'label'
sounds like the imperative form, i.e. to label an issue.

Add limit flag for issue and release commands

If the result limit is N, the default `per_page` will be `N + N/2`
instead of the default 100. This aims to achieve faster API requests for
relatively low values of N.

- Stops HTTP pagination once the limit has been reached
- Cleans up client implementation by passing around a filter block

Fixes #1379

Remove go-octokit dependency

`dep` aggressively prunes everything under `vendor/`. Unfortunately,
this consistently nukes the contents of `vendor/bundle` directory.