Add optional harmony support to browser transform

This mirrors the react-tools API; it doesn't include support (yet) for transforming `<script type="text/jsx">` blocks with the ES6 transforms, mostly because I don't have a good API in mind there.

Test Plan: Ran

    JSXTransformer.transform("var two = () => 2;", {harmony: true}).code

in Chrome's dev console and got back some ES5 code.

cc @Daniel15 @cpojer