Port plugin to ES6

Proof-of-concept attempt to convert this plugin from AMD to ES6 module.

Note:
* As I was going to reindent the code, I ported it to 4-spaces, which I think we wanted to make standard across the codebase?
* The output AMD module (from plumber build) looks a bit funky, with ES5 getter based defaults. From what I can Google, that should be [supported by all modern browsers](http://kangax.github.io/compat-table/es5/#Getter_in_property_initializer) (but excluding IE7-8).
* It'd be good to prove that the output of `traceur.toCommonJs()` works fine for people who live in browserifyland.