Remove _sync

* Calls `Model.save` instead of `Model._sync.save` for proto methods.
* Maintains backwards-compatibility with _sync plugins.
* Removes Model.find, Model.all, etc. in favor of a reference implementation [memory sync plugin](https://github.com/alexmingoia/modella-memory).
* Adds [documentation](https://github.com/alexmingoia/modella/#storage-layer-plugins) for sync plugins.
* Update tests.