iframe support, AMD and CommonJS support 

https://github.com/dexteryy/buzz/commit/eaecfd272e19415a30fce91aa05b7dffd4f24bf2

With new option `document`, we can create <audio> element in an iframe's context, then we can truly destory useless audio through removing or reloading the iframe. This is useful for avoiding some crash bug in Chrome. 

https://github.com/dexteryy/buzz/commit/248d07b724d7eb23e6db944b7d46aab84ed48888

We can use buzz.js as an AMD module with [require.js](http://requirejs.org/) or [oz.js](http://ozjs.org), without any configuration or hacks.