Add clock problem

Adds the clock problem from the other tracks. Is there a common standard to be used for the tests/examples, if not may I suggest http://www.php-fig.org/psr/. I will be happy to move the existing problems over and adjust the makefile to allow for the files to be renamed to the more commonly used form Class.php ClassTest.php

Let me know if there are any problems with this PR and I'll address them.