Bitbucket post-receive hook

This is a Bitbucket post-receive hook that will automatically close review
requests as "submitted" after a push. To determine which review requests should
be closed, it scans through each commit's commit message for the following
strings (case-insensitive): "Reviewed at <reviewboard_url>/r/<id>" or "Review
request #<id>". The regex used for this can be overriden in settings_local.py.

Testing Done:
Basic testing on a git repo performed. Test passed successfully.

All tests carried out on both mercurial and git repos.

Tests:
1) Close an open review request.
2) Try to close a review request that already has been closed as submitted.
   (Logged a warning saying that the review request has already been
   submitted).
3) Try to close a discarded review request. (Marked the review request as
   submitted)
4) Push without a review request id in commit message. (Error logged that the
   request id does not exist).
5) Push with a non existent request id in the commit message. (Error logged as
   Review request id does not exist.)

Reviewed at https://reviews.reviewboard.org/r/5653/