* Presentation change: if you run a program without arguments,
  runs no-argument `help` instead of `usage`.  This prints out
  both usage information and a list of commands, which seems more
  useful.  That's how most modern programs do it (e.g. `git`, `hg`).
* Minor API change: renamed Appeal's custom exceptions, to
  remove the word `Appeal`.  So, for example, `AppealUsageError`
  is now simply `UsageError`.  I added aliases so the old names
  still work; I'll remove them eventually, but I promise to
  keep the old names around for at least one year.
* Fixed usage generation, added tests.
* Fixed a bug where using a "simple type" (e.g. bool, float)
  as an annotation for an option would cause Appeal help to
  raise an exception.  Fixes #15.