Provide Callback functions for PrettyPhoto open and close events.

Modified jquery.prettyPhoto.init.js to provide callback functionality to open and close events. Supports "generic" open and close events (wc_pp_close()/wc_pp_open()) or functions customized to the "type" of overlay (i.e. wc_pp_review_open()/wc_pp_photo_close()).

These functions can be defined in external javascript files (such as themes or plugins) to provide extended functionality to the WooCommerce prettyPhoto overlay system.

Example use cases would be to trigger analytics tracking events or dynamically setting things like a title or image description through the use of these callbacks.