Gitlab support

**What does this PR do?**
It adds support for the GitLab webhooks. Currently it does support: Issue hooks, Merge request hooks and push hooks.

**Relevant links**
GitLab documentation about webhooks: https://github.com/gitlabhq/gitlabhq/blob/master/doc/web_hooks/web_hooks.md

GitLab feedback item: 
http://feedback.gitlab.com/forums/176466-general/suggestions/5785859-add-gitter-support