Add migration creation helper and enforce South 1.0

This crude and destructive script helps create migrations for both South
and Django's native migrations framework. I'm happy to leave plenty of
room for improvement so that the King of Bash (aka @codeinthehole) can
do his thing.

This commit also enforces South 1.0. It's a backwards compatible release
which explicitly is meant to help the migration; it makes both our and
our users life's easier. Notably, it first looks for a south_migrations
directory before falling back to the migrations directory in an app.
That nicely avoids any conflicts between migration files between the two
migration frameworks.
Drop support for Django 1.5

Oscar's policy is to support two versions of Django. So while there
isn't a major reason to drop support for Django 1.5, dropping support
for Django 1.6 once 1.8 is released will reduce complexity considerably.
fork_app now copies both migrations directories

I think the Oscar upgrade/forking process for forked apps will get
simpler with the new migrations. One probably will be able to just fake
an initial migration, or something like that. But for now, copying
them across as we did before should be a safe bet.
Add new-style migrations

These are created with the migration helper script.