Added greaterThan(x) invocation matcher

This is something that I would have a use for and so I thought others may have as well. I have not yet added an alias method to PHPUnit_Framework_TestCase since it's in a different repo. If this is something you have interest in, I'm also happy to add GreaterThanOrEqualTo, FewerThan, and FewerThanOrEqualTo matchers as well.