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
I'm afraid with the latest code in the development branch a lot of this won't work anymore. I'm trying to pull as much functionality out of the provider specific part and move it into the generic `base.rb`. I'm aiming at refactoring these provider classes into a set of lean methods that basically just create instances of git-review's models (e.g. `Request`, `Commit`, ...). If you take a look at the current implementation in `github.rb`, you might be able to see where I'm going with this. In the current state it's far from finished, though.