Added Arr::count() helper

This fixes issue #4516 (http://dev.kohanaframework.org/issues/4516)

I did the second option for an unexpected parameter, but used a Kohana exception rather than creating an UnexpectedValueException (or similar). I figured an unexpected value exception might be a bit weird in the Kohana filesystem in terms of needing a exception file in a value directory in an unexpected directory. At a loss for a better approach I went with a Kohana exception. Happy to implement suggestions on that count.