The .pyc bytecode files have burned us too many times when switching
between revisions of Review Board. No more. This adds a new alias to
the Review Board repository so that we can wipe out all pyc files in
one fell swoop.

Example usage:

rbt clear-pycs

Ran 

find . -name "*.pyc"


locally, and saw many pyc files.
Ran

rbt clear-pycs.


Re-ran

find . -name "*.pyc"


Didn't see any pyc files listed.