Initial pass at new style Posts Controller 

Initial Controller routes and methods:

- [x] get all posts
- [x] get a post
- [x] get revisions for a post
- [x] create a post
- [x] update a post
- [x] delete a post

Test coverage routes and methods:

- [x] get all posts
- [x] get single post
- [x] get revisions for a post
- [x] create a post
- [x] update a post
- [x] delete a post
We should have a base; this is something we kind of screwed up with v1. Please please please do not call it `Post_Type_Controller` though; a "post type" is a categorisation of an individual post. This would be similar to calling a common parent of `Category_Controller` and `Tag_Controller` `Taxonomy_Controller`, when really it's `Term_Controller`.

There's no real equivalent terminology here apart from "post", alas. I'm not sure what we would call it, just that `Post_Type_Controller` would be bad. :)