Added unit testing skeleton with mostly stubbed server tests.

This is a start to resolving #6 with a skeleton set of unit tests, but certainly needs a lot more love.

I've mostly outlined what I think is probably a good set of minimal tests for WP_JSON_Server at least. I'll be sure to also jump into CTP, Media, Pages, Posts, ResponseHandler, and Taxonomies later here.

Some notes:

* The Travis CI configuration uses my personal mirror of the develop repo (since there still isn't an official one), but really just because it's more work to deal with the logic of checking out SVN trunk versus tags for specific versions. This shouldn't matter though since this is a temporary setup that would be removed when merged to core.
* I've left a bunch of notes in the inline comments in numerous locations and todos. Since it's mostly stubbed, they're probably better off there for when someone (not just myself) gets around to writing these tests.
* The instructions for running the unit tests locally are simple: checkout develop.svn (or use my git mirror as well), configure the core unit tests as usual (wp-tests-config.php in root), install WP-API into `src/wp-content/plugins`, and run `phpunit` from the plugin directory. See [tierra/wordpress-plugin-tests](https://github.com/tierra/wordpress-plugin-tests) for more details if you like.