Add /taxonomies routes

As described in #198, 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+)`

**Details**

I have implemented this by augmenting the existing `get_taxonomies`, `get_taxonomy`, `get_terms` and `get_term` methods, rather than creating new ones. I re-ordered the arguments of these methods and set a default for `$type = ''` in the arguments for all route handlers to allow the same function to be used with or without a `<type>` parameter.

If the type if `''`, then, `get_taxonomies` is used to retrieve all the registered taxonomies (for the base route), and the `$base_url` value is updated to reflect the new routes (for all endpoints accessed through `/taxonomies/`.

Note that at present this does *not* contain the `types` cross-linking described in #198.

Fixes #198, fixes #197.
Should we use `$context` here as well, instead of `$_in_collection` and `$_in_taxonomy`?