Add a draft for indexes.

Some draft API specifications for custom indexes. Key points:

- Introduction of `django.db.models.indexes`
- Support for custom classes to `db_index`
- Introduction of `Meta.indexes`
- `spatial_index` would be deprecated and a corresponding chunk of code removed from gis, allowing fields to specify their own index type, via `Field.get_default_index`
Hmm, I don't intend to try to write this before the changes to expressions. It also would ideally support `transform='field__transform'` or similar as well. This should be a little easier to extract the sql from I think.