Including element resize watcher to deal with hidden blocks

The directive didn't work for initially hidden blocks (i.e. dropdown menu), because isOverflowed() returned false due to clientHeight being zero at the time is it called. I have added a resize watcher for the element itself, that is called when it is displayed and clientHeight gets its intended value from CSS. This watcher could replace the window resize one, I leave it to your decision.

Thank you so much for this awesome directive.