Abstract core resource methods to a base class.

[tdstein]

Currently, each resource implementation defines the same methods when available (find, find_one, create, etc.). Initially, it didn't make sense to abstract these methods into a base class since it wasn't clear how much overlap existed. But now, it's obvious that a majority of implementations between API endpoints are identical. In cases where endpoints differ, the subclass should override the base class. When the method is not supported a NotImplementedError should be raised and the method should be documented as such.

[tdstein]

The core components of this feature were introduced in #314

We will continue adding this functionality during development of other API endpoints.