Add /taxonomies routes

It is frequently desireable to know about all taxonomies, regardless
of the post type to which they are bound; it is also more convenient
to be able to retrieve taxonomies from a base-level endpoint even
with the default post types and taxonomies.

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