Fixes #127: Add unit testing for Validator

@bserdar Here are the unit tests we talked about using Mockito. Let me know what you think.

I am not totally happy with the tests for ConstraintValidator, but it is a start. I am exercising most of the code, but the unit tests are not asserting anything. This is primarily because with the Constraint implementations being mocked out, ConstraintValidator really just becomes a driver for the functionality.