Add Unit Testing Suite

These commits add a basic unit testing framework for WC using PHPUnit and the WP Unit Tests library, along with automated testing via travis-ci. I've added completed tests for the webhooks REST API endpoint as a reference implementation for other tests to follow. There's a basic readme with steps to install and run the tests [here](https://github.com/maxrice/woocommerce/blob/feature-3176-unit-tests/tests/README.md).

The phpunit.xml config is set to automatically generate an HTML test coverage report like:

![screen shot 2014-09-01 at 4 16 22 pm](https://cloud.githubusercontent.com/assets/1579862/4111902/6a77a9fa-3216-11e4-9923-3d97b14a7a9e.png)

Now we just need everyone to start writing tests :smile_cat: 

@mikejolley I don't have admin permissions for the WC repository so you'll need to log into travis-ci.org and connect the repository to have automated builds.

Closes #3176