Change all 'callback' typehints to 'callable' to make the code PHP 5.4 friendly.
Change all 'callback' typehints to 'callable' to make the code PHP 5.4 friendly.

Reference:
 - http://php.net/manual/en/language.types.callable.php
 - http://php.net/manual/en/function.is-callable.php
 - http://nl.php.net/manual/en/language.pseudo-types.php#language.types.callback

Xerkus/zend-mail,basz/zend-mail,mleko/zend-mail,matbech/zend-mail,rapidsoft-de/zend-mail,stefanotorresi/zend-mail,MadCat34/zend-mail