Use AMD loading with fallback to global vars

Hello,

I'm porting a project to use RequireJS so I added AMD support to this jquery plugin that I've been using. The change is unobtrusive and backwards compatible, so I hope you'll consider bringing it into the mainline.

I got the basic pattern from here: https://github.com/umdjs/umd/blob/master/jqueryPlugin.js

- Kenn