Added assertion to check if Traversable contains instances of a given classname

I had a situation the other week where I needed to check if an array or traversable object contained an instance of a given class. After digging around I found that PHPUnit doesn't seem to have an easy way to do this so I created this pull request to add that functionality. It allows a user to assert that an array or traversable object contains at least X instances of a given classname (where X is an integer, defaulting to 1)  or that an array or traversable object contains exactly X instances of a given classname. There's also an assertion to assert that an array or traversable object contains no instances of a given class.