- Test everything!
- Get rid of as many global variables as possible
 - Make them all attributes of Adventure and/or Game?
- Remove fields that are always the same when saved (gaveup, foobar, ???) from
  `Game` (or just from its pickling method?)
- Merge the Adventure and Game classes together?
- Ensure the code works in both Python 2 and Python 3
- Add docstrings
- Use constants in place of as many magic numbers as possible/reasonable
- Add an option for enabling an "old-tymey" interface in which all output lines
  (including the MOTD!) begin with a single space and there is no "> " input
  prompt?  (Lowercase letters will still be used, though.)
