Put queue into a top-level package

Under rare conditions which are strangely common in the openedx
universe, you can find yourself trying to import Queue into boto and
instead importing ./queue.py, which breaks everything.

This fixes the conflict by renaming queue to queue_xqueue and also by
putting into an openedx_certificates package.

Eventually, everything should migrate into openedx_certificates so that
edx-certificates can be pip installed. queue was renamed with the
queue_xqueue style to imply that there may be other queue
implementations in the future.
Merge pull request #24 from edx/jrbl/package_queue

Put queue into a top-level package