Expose more information about open merge requests

Created by: calmh

This adds a new API call per project that returns all open merge requests, including notes and commits. We use this to automatically evaluate merge requests for style etc and post a +1 or -1.

Since the existing merge requests call returns all MR:s (even those already closed or merged) and the new information I want to return might be expensive, I chose to add a new API call for only open requests instead of fleshing out the existing one.