Trivial spelling: s/you/your/.
Add support for GitLab notifications.

This commit adds support for GitLab notifications (push webhooks) available at
the url endpoint: '/hooks/gitlab/'.

Summary of the changes:
* Add the url endpoint and the appropriate gitlab handler method.
* Add to 'Notification hooks' on data/<project>.
* Add tests.
* Update documentation.

This commit has been tested with GitLab CE 7.1.0.
Check for the content-type instead of the service name.
Merge pull request #500 from nblock/feature/gitlab-support

Feature/gitlab-support