Added a new `game_date` field to plays to make sorting them easier.
Make sure plays/drives are sorted by id when initially inserted.
Get rid of unused exception value.
Simulate games for testing!

- Added a "simulation" mode to `nfldb-update` to simulate a
  game being played. It's a bit rudimentary, but it works.
- Also moved nfldb-update into a module. The script is now just
  an argparse shell.
version bump to 0.2.5