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.

@sarina notice that this fixes one of the things we ran into and documented in Troubleshooting. But I think we should leave that note in place anyway. :-)

@stvstnfrd, you asked for it.