Update Exceptions

Fix a syntax error in the InvalidArgument exception, and extend it with another exception called InvalidRule.
Update PHPCI Configuration

No need for extra config for PHPUnit when the configuration already exists in the phpunit.xml file.
Add Replacement Class

Added the alpha version (untested) of the class that will eventually supercede When\When for version 1.0, but it's time I went to bed so here's the commit.