Changed how jQuery.on() calls were being bound.

Rebound jQuery.on() handlers to the document so they will behave as jQuery.live() did. See StackOverflow post at http://stackoverflow.com/questions/8065305/whats-the-difference-between-on-and-live-or-bind for a decent explanation of binding with .on().

Note: this commit changes the type of piggybak.shipping_els from a jQuery object to a string as that is what jQuery.on() takes as a param. I noticed this change affects piggybak_coupons (at least until my related pull request gets merged in). It may affect some of the other extensions as well. Not sure if you'd prefer using a separate variable for the string.