Implement: Add unit tests

Added initial unit tests (8 tests with 22 assertions).

![screen shot 2013-10-26 at 4 40 05 am](https://f.cloud.github.com/assets/78313/1411752/15ab4dde-3dbe-11e3-8314-40b88c4d939a.png)

Should fix #25. Please note that the tests check expected hooks this plugin uses, required classes (which extends `WP_Customize_Control`), and enqueued scripts. Testing the previewer involves testing the AJAX request which needs more consideration.