Support requeuing proposals after a merge failure

Look for reports that the merge failed when seeing if a pull request is
mergeable, and obey merge directives following it. The previous scheme
required all users to be admins to remove the 'merge request accepted'
messages from the bot to get it to retry.
Merge pull request #14 from bz2/mergable_requeue

Support requeuing proposals after a merge failure

Look for reports that the merge failed when seeing if a pull request is
mergeable, and obey merge directives following it. The previous scheme
required all users to be admins to remove the 'merge request accepted'
messages from the bot to get it to retry.