Expose branch merge proposals through launchpadlib

I would like to be able to access merge proposals via the API as well. I'm more interested in the review side of it, though.

What I want to do is:

  * List all open review requests (both filtering on reviewer, and get all open reviews for a project)
  * Get basic information about the review request, status, comments, existing reviews, who added the merge request, which branch it is, the diff (if available).
  * Manipulate meta-data. More specifically, I want to vote and change the status of the merge proposal. But also change the whiteboard and add comments.

Just as an update-

  - Branch merge proposals can be fetched from the branch or from the project (and filtered in the project by status)
  - Branch merge proposal comments can be read
  - I have a branch that allows one to review and add comments.
  - Tim exposed the functionality to update the diff between the two branches.

The only thing on Bjorn's list that I don't have on deck for me is to filter on reviewers and get all proposals for a reviewer.

The whiteboard won't be exposed, as it sounds like we're looking to remove the whiteboard completely (it's not exposed on the webui anymore).

The final part of this (getting a person's submitted merge proposals) landed in rev 7902