Automated Test Suite

I've started working on an automated test suite for Ardent using PHPUnit and Mockery (same as used in Laravel).

Since [`LaravelBook\Ardent\Ardent`](https://github.com/laravelbook/ardent/blob/master/src/LaravelBook/Ardent/Ardent.php) is a large class, I've decided to split the tests out into separate 'feature oriented' test classes.  Currently, I've written most of the 'Validation' feature's tests, and have staged test classes for auto-hydration and model hooks.

I wanted to present this before I got much further in writing the tests to ensure that it's in line with the project's style and architecture (there don't seem to be any contribution guidelines). __Please do not merge this yet__, but please do comment on the changes, and try running the tests yourselves.