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
Any reason for changing this? Personally, I don't think that `full_name` makes much sense unless there is something like `short_name` or `name` to distinguish from it.