Add unit tests for new taxonomy convenience methods

This presumes the following aliases are defined on the WP object:

- `.tags()`: Create a taxonomies request pre-bound to 'post_tag'
- `.categories()`: Create a taxonomies request pre-bound to 'category'
- `.taxonomy('tax_name')`: Create a taxonomies request pre-bound to 'tax_name'

See #63, #64, and #65
Add wp.taxonomy('taxname') method for getting tax objects

Note that per the comment in #65, this returns the taxonomy resource, not its terms
Implement wp.categories() and wp.tags() (see #63, #64)
Document new taxonomies shortcuts in the readme

Fixes #63, fixes #64, fixes #65