South compatiblity issues

Hello,

I'm having problems while using South to remove some unique constraints. It seems that South builds a constraints map through a query that is always relying on a specific fixed schema (public, in this case).

This problem was already reported in some time ago [here](http://south.aeracode.org/ticket/510) (south specific).

**The solution:** At first, South tries to check if `settings.DATABASES[<dbname>]` specifies a schema otherwise it will fallback to a default "public" schema. So I think we could set the database `SCHEMA` every time schemas swap, so South could rely on it.

I'm wondering in implementing this at `DatabaseWrapper` because it provides access to the settings through `self.settings_dict`. I made some tests and it works perfectly. It's a small change.

What do you think? Can I make a pull request?