Updates to RichTextAreas

* Add the "code" plugin, which WP pulled (#122)
* Add ability to set toolbar buttons when constructing a `Fieldmanager_RichTextArea` object (#44)
* Fix some bad HTML
* Add basic test coverage for the field, plus test coverage for the above updates
No reason to change, but just for info you should be able to use `$this->factory->post->create_and_get( array( ... ) )` for the same results.