Fixed a naming/logical issue and added a bi-directional relation between `Term` and `TermTaxonomy`

I fixed a naming/logical issue I introduced myself with 9101a9b40e2f83bb7b1bb1bda5624f3f9a55386a. When needing Wordpress Tags you probably want to get back instances of `Term` but if you want to get the post categories you need `TermTaxonomy` instances because you may need the parent category.

Also made the `Term` -> `TermTaxonomy` relation bi-directional.