Quick attempt at a Rules subapp.

Addresses #100.

   * We use `django-markupfield` to choose a markup language. So far,
     I've enabled markdown and plain-text (plain-text adds
     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. That addresses #15.

   * The tags are correct, but there's no styling to treat them differently yet, so the effects aren't really visible outside of the HTML source.