Add /taxonomies routes

It is frequently desirable to know about all taxonomies, regardless
of the post type to which they are associated; it is also convenient
to be able to retrieve taxonomies from a base-level endpoint.

This adds the following routes to the taxonomies class:

- /taxonomies
- /taxonomies/(?P<taxonomy>\w+)
- /taxonomies/(?P<taxonomy>\w+)/terms
- /taxonomies/(?P<taxonomy>\w+)/terms/(?P<term>\w+)

Bug Reference: #198