Add check_assets, check_media, and clean_media manage.py commands

Commands

* check_media - Checks that Files in the media directory exist as Assets
* check_assets - Checks that Assets exist as files in the media directory
* clean_media - Clean Roundware Media directory by organizing mp3/wav into project_id subdirectories

Usage:

    (roundware)vagrant@roundware-server:~/roundware-server/roundware$ ./manage.py check_media