Iterators\Filter: count() counts only fitlered items

Fixes behaviour of count() method - now it counts only filtered items, not all items
This class is intended to be removed as soon as minimal requirements of Nette will be PHP 5.4. It is just polyfill for CallbackFilterIterator. 

I did not understand your comment about the function __call. If original CallbackFilterIterator has not __call or count, this polyfill shoun't have it too.

Btw, calling iterator_count is really not good idea. Some iterators are countable, some not.  And iterator_count will not return iterator to its previous state. The fact that PHP returns 1 for `count($noncountable)` is pain in ass and should be reported as PHP bug.