Add gitlab webhook based on existing github webhook.

I'm currently using this webhook in my buildbot environment with my gitlab (http://www.gitlab.org/) server. It works pretty well.

The change I made is pretty simple. Since gitlab send out webhook in simillar format to github, I just have to make few changes to comfort the gitlab format. Changed files detection is not working because gitlab doesn't send this information.

There is no test at the moment. But I'll be more than happy to contribute some tests if we're planning to merge this PR in.