Model Plugin and Theme Resources with get_item_schema()

[danielbachhuber]

To explicitly declare the expected format of the Plugin and Theme Resources, they should be first modeled with get_item_schema().

The prepare_item_for_response() method should use these schemas to determine what data is present on the final response.

[BE-Webdesign]

The initial modeling of the schema is in #24, and prepare_item_for_response() is already in.