Refs #54 and #55, adds an abstract TestEnvironment class and a UnitTestEnvironment stub.

The tests/unit-tests.php script will create a test environment, run some unit tests, and then tear down the test environment. TestEnvironment will also be subclassed in #55 as an environment from which to input a lot of fake test data. The TestEnvironment's get_word() function can be used to get a random word from words.txt.