Phpunit framework

This adds a phpunit testing framework as per https://github.com/wp-cli/wp-cli/wiki/Plugin-Unit-Tests.

To use:

* Have a WordPress database that you don't care for, but with WPeC installed
* Run bin/install-wp-tests.sh <db-name> <db-user> <db-pass>
* Run phpunit
* Revel in the sea of green.
* Realise there are many more tests to write.