Added 'onSnap' callback and 'onSnapEvent' DOM Event

Hello Benoit Pointed,

First of all thank your for your small but very useful jQuery plugin.

I have implemented two small changes that allow one to be noticed of a 'snap' events. First a 'onSnap' callback which is called when an element is snapped. It can be set alongside the other options. Second a DOM event which is fired when an element is snapped. To avoid to many redundant events I have copied and used the debounce function from underscore.js library.

Keep on the good Work!
Crunc