Populate the Bugs on a new review request with push2rb's reviewid.

push2rb sends along a reviewid to Review Board when it creates its review
requests. For Mozilla, the reviewid is just the Bugzilla bug number. This
patch makes rbbz verify that the passed reviewid is indeed a valid and
accessible bug number, and then sets the review requests Bug field to
that reviewid.
Allow applying bug IDs to child review requests, and mirror comments.
Merge pull request #3 from mikeconley/create_bug

Make the Bug field get populated on publish from push2rb's reviewid.
Add push guards to only process relevant review requests