Quick attempt at a Rules subapp.

Main cool things:

   * We use django-markupfield to choose a markup language. So far,
     I've enabled markdown and plain-text (plain-text will still add
     urls and paragraph tags, so it's still pretty cool).

   * With markupfield and markdown, we now have third party
     dependencies... it seemed like a good time to get a
     requirements.txt file set up.
Remove BaseRule from the database.