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
Oh, good catch. I've been working quite a bit on getting rid of all generic things from`github.rb`. Thereby finding a couple of similar issues. I'm gonna put this on my list and try to add it to the model as soon as possible.