Allow use of native migrations in 1.7 — Take 2

For #1559 take 2. Replaces #1653.

(The branch name was obviously causing GitHub some confusion...)

Merges against 2.4.0 branch (since it's a small breaking change).
Again, a change of emphasis to try to make it more clear who exactly is affected.  "If you're using `south` for your database migrations, you'll need to make sure you're using version x.xx and include the following in your settings: ...<code>... This change is required in order to allow south to lookup the older style south migration files, instead using the newer native Django migration files, that are supported from Django 1.7 onwards"