On second thought, don't require a city to be associated w/ a site.
Added a 'slug' field to city.

I also discovered that using call_command() to import fixtures in a
migration, as the South docs suggest, doesn't work when the model
loading the fixtures changes, since it's using the latest version of
the model rather than South's serialized orm object. So I had to
create the fixture in code instead, which is honestly more readable
anyways.
Merge branch 'multi-site' of github.com:toolness/hive-django into multi-site
changed chicago.json to have a city slug, and removed its site model.