Moved to class-based unit testing framework

Test cases are subclasses of TestCase. They may include a .set-up method, which is run before each test in the test case, and a .tear-down method, which is run after each test in the test case. All other methods in the test case (conventionally named test-<method>) are run as separate tests.

To run all tests, include the tests/Test.yak file and invoke run-all-tests. I use this command:

```sh
yak library/*.yak tests/Test.yak -e run-all-tests
```
Merge branch 'master' of github.com:williamtmurphy/Yak