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
actually i reverted this but there is an issue here:

`owner.login` is not part of the model, it works with github because you pass a github object to the `owner` field that happen to respond to the `login` method.