RestModel v2.0.0

This PR will eventually have RestModel v2.0.0 in it.

- [x] Clearer use of `attrs` or `attr`
- [x] `save`, `delete` functions update the cache
- [x] Simpler parent model API
- [x] Differentiate `$.ajax` options from `RestModel.ajax` options
- [x] Separate caching logic from AJAX logic