Add support for multiple players. Closes #13.

- Remove unique constraint from Packet.
- Filter by player_name in scheduler.
Add player_name support to PlayHistory.
Remove embarrassing semicolon from Python code.
Configure Beats to be run as a daemon.
Compare Unicode paths properly. Fixes #31.
Add command line tool for adding songs by NetID.
Escape password string.
Add play counts to frontend. Closes #36.
Re-enable SQL connection pooling.

Setting max_overflow to -1 (no limit) to prevent SQL connections from
hanging indefinitely on a large number of concurrent requests.