Allow use when jQuery is running in No Conflict mode

Currently, the library can't be used when jQuery is running in [No Conflic'](http://api.jquery.com/jQuery.noConflict/) mode, as `$` is referenced from the global scope.

I've wrapped the code up in an anonymous function so `$` always contains `jQuery`.

Just as a note, I ran into this when using the library on a Magento site, where Prototype is included as well as jQuery.