Converted existing South migrations to Django migrations

[frontendr]

Followed the directions given in the Django documentation for upgrading from South. Deleted all migrations and created new ones. Importing the TranslatableModels using get_model results in a model with the default manager and so I've imported the models manually.

[Tyrdall]

@frontendr Thanks a lot! Can you please move the old south migrations into a new folder instead of deleting them? Otherwise users using Django<1.7 won't be able to keep this app running.

Then we should add a note to the README. Something like:

If you are using a Django version below 1.7, add the following setting::

SOUTH_MIGRATION_MODULES = {
    'image_gallery': 'image_gallery.south_migrations',
}