get_model / field_types refactor

[arthanson]

Refactor, cleanup and simplify a lot of the code around get_model and field_types:

• Separated out get_serializer_class from get_model to avoid recursion issues
• Added init.ready to register the serializers on startup
• Removed generating_models stuff in field_types and models, with a previous change to prevent circular dependencies and the change to get_serializer_class this should no longer be needed. This simplifies and cleans up the code quite a bit.
• removed fields, manytomany_models, no_cache, _generating_models params from get_model. With the code cleanup these should no longer be needed. no_cache was removed as it shouldn't be needed and cache should always be used.
• views had a set of change to call the get_serializer_class as it is no longer being called in get_model.