remove need for jQuery global

e.g. by wrapping plugin like this:

``` js
(function(factory){
    if (typeof define === "function" && define.amd) {
        define(["jquery"], factory);
    } else if (typeof exports === 'object') {
        factory(require('jquery'));
    } else {
        factory(jQuery);
    }
}(function($, undefined){
  // the plugin code
}));
```
