* Add new product attributes endpoint that allows searching.
Including custom attributes in the results.
* Add endpoint to fetch custom attribute values by slug.
* Add initial tests for Product Attributes endpoint.
* Test Product Attribute search.
* Fix schema/params for attribute terms endpoint.
* Add tests for custom term values endpoint.
* Add a LIMIT to the attributes query.
* Add endpoint to retrieve single attribute by slug.
Fixes the 'self' link.
Jeff Stieler