This includes the beginnings of the new base class for rb commands,
and the rb post command.

'rb post' is meant to eventually replace post-review for the common
task of generating a diff and creating/updating a review request. It
has many of the extra options of post-review stripped out, to make
it more focused.

"rb <command>" now uses the new entry points, expecting subclasses
of ``Command``. If the command isn't found it will attempt to
execute "rb-<command>" on the system.

Local testing with git repositories. This review request was posted using 'rb post'