Gitlab integration development

This follows #91

Please review the following changes:

This was quickly written to work with less features than the github version (namely: comments)
No tests yet so you don't have to merge this if you don't feel like it
It works with any gitlab server (when the string 'gitlab' is in the hostname) using Gitlab API v3
It uses the gitlab gem
Is this really the same? Assuming the repository has multiple contributors, wouldn't it be possible that the latest commit has been made by someone else than the repo owner? In that case we'd end up with the wrong URL for the request.