Refactor some parameter handling.

Now raise an error if a bad generation parameter is provided. For
consistency, add the min_generation parameter to /children and /point.
Fixes for Django 1.7.

Also make sure all tests run in Django < 1.6, and with a
non-4326 SRID setting.
Migrations for Django 1.7.

Move South migrations to south_migrations, and add a warning to update
your settings if you try and run migrate with Django<1.7, South<1.0.

Thanks to http://treyhunner.com/2014/03/migrating-to-django-1-dot-7/ for
the idea.