migrate from south to django 1.7 built-in migrations

Couple things here:

1) remove the south dependency from setup.py, since that prevents it from
   being installed in a 1.7 project that doesn't have south
2) moved south migrations to south_migrations directory. South 1.0+ looks
   there first. So this app will still work with <1.7 and south, but
   it now requires South 1.0+. Depending on how long we want to maintain
   dual support, we just need to remember to make migrations for both paths
3) new Django 1.7 migration in migrations/

I'm going to bump the version number next, to 1.0.0 to signify that this
is a (potentially) not backwards-compatible release (since it requires South
to upgrade to 1.0+ if you are on less than Django 1.7)