Feature/gitlab-support

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.
* Trivial wording spelling mistake.

I tested the feature with an in-house installation of GitLab CE 7.1.0.

Please review the proposed changes and give feedback. I tried to catch all places, where a new notification hooks needs to be mentioned, but I might have forgotten something.
Maybe it would be better to do this decision based on content type rather than hardcoding service name here.