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`
We have the same issue here as with custom lookups and fields w.r.t 3rd-party backends. We should provide easy ways for both backends to specify index support and indexes to specify backend support.