Support v2 Taxonomy Routes

[kadamwhite]

Support default taxonomy collections using v2 endpoints.

Many of these changes are adapted from work by @jasonphillips in his fork of the project -- full props to him for pioneering how to adapt the existing infrastructure to the new endpoints.

Tasks before PR can land:

• Update library to support querying /taxonomies endpoint for available taxonomies
• Update library to support querying /taxonomies/:taxonomy endpoint for taxonomy definition object
• Update library to support querying /categories 1st-party taxonomy term collection
• Update library to support querying /categories/:id individual category terms
• Verify library supports querying for individual category by slug
• Update library to support querying /tags 1st-party taxonomy term collection
• Update library to support querying /tags/:slug individual category terms
• Verify library supports querying for individual tag by slug

Related work, to be executed separately:

• Update library to support querying for the terms associated with a post (skate to where the puck will be: depends on Move term->post relations to the posts controller WP-API#2063)
• Verify (via integration test) that library works with hierarchical and non-hierarchical custom taxonomies

[kadamwhite]

Querying by slug is "supported" inasmuch as 23ae892 adds a demonstration of how to do this by combining ?search and iterating over the responses. That's the best we can currently do due to limitations around what the API provides for taxonomies.