add zepto compatibiliy

Changing the way we extend $.fn with each2() function now select2 should be compatible with Zepto.js (http://zeptojs.com/).

Besides zepto.js it also needs data.js (https://github.com/madrobby/zepto/blob/master/src/data.js).

Zepto.js + data.js minified and gzipped are below 12kb. I think this is around half the size of jQuery.

Zepto doesn't support IE but it's great for mobile devices with modern browsers and bad connections.

Is there any test suite I have to pass to get this merged?

Thanks!