Added  Events(open_before and open_after) to Dropdown javascript

A very minor change to add two events to Dropdown javascript. A solution like this has been asked for a number of people on stackoverflow.
http://stackoverflow.com/questions/836694/jquery-adding-change-event-to-dropdown  

These two events are triggered on the open action and differ on whether they occur before or after _toggling the parent class to "open"_