Refactor to work in node.js as well as browser

Enables radiotag.js to be used from node.js so that tagging doesn't have to be done in the browser. 

Use in node like `var radiotag = require('radiotag.js')`.

Moved from require.js style to [browserify](http://browserify.org/). The Grunt task still builds to `dist/radiotag.js` which exposes itself as either a window global, an AMD module or a commonjs-style module.

I've tested this with the [cpa-client device web app](https://github.com/ebu/cpa-client) and tagging still works.