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
Fix --output-file option

When opening the file for binary writing, the file mode was incorrectly
specified.

Additionally, the command line options have been updated to be more
intuitive:
- long: `--report-file`
- short: `-r`

The file can now be successfully written with:

python create_pdfs.py \
    --report-file report.csv \
    --course-id 'edX/DemoX/Demo_Course' \
    --no-upload \
    --name 'Guido'