Added CallbackFilterIterator and RecursiveCallbackFilterIterator classes

Added CallbackFilterIterator and RecursiveCallbackFilterIterator classes

[DOC] This is a concrete FilterIterator which takes a callback as
constructor parameter, and uses this callback for filtering. This allows
to use FilterIterator without extending it.

CallbackFilterIterator Example:

$it = new ArrayIterator(range(1,100));
$it = new CallbackFilterIterator($it, function($value) {
	return $value % 2;
});
foreach($it as $value) // ...

RecursiveCallbackFilterIterator Example:

$it = new RecursiveDirectoryIterator("/");
$it = new RecursiveCallbackFilterIterator($it, function($file, $k, $it) {
	return $it->hasChildren() || $file->getSize() > 1024;
});
foreach(new RecursiveIteratorIterator($it) as $file) // ...

The callback takes the current value, the current key and the inner
iterator as parameters.