Spike: Transpile to System.register

Alternative to #120. See discussion first.

Depends on [systemjs](https://github.com/systemjs/systemjs)'s [`System.register` shim](https://github.com/systemjs/systemjs/blob/master/lib/system-register.js).

**ES6 syntax**

``` js
// app.js
import $ from "jquery";
import _ from "underscore";

function start() {}

export { start };
```

**System loader output**

``` js
System.register('app', ['jquery', 'underscore'], function() {
  var $ = System.get(arguments[0]);
  var _ = System.get(arguments[0]);

  function start() {}

  return new Module({ start: start });
}
```

* [ ] Deal with anonymous module names where `this.moduleName` is undefined
* [ ] 50% Complete test suite

/cc @guybedford @domenic @stefanpenner @caridy @wycats