Fix 1530 - Asynchronously load scripts with JSX transformer

I was kind of curious about how this script worked, so I took a stab at #1530. The implementation is fairly simple, an enhancement would be to load all of the script tags in parallel and then `run` them sequentially.

But, this gets the waterfall without synchronize requests on [an example I had](http://www.natehunzaker.com/d3-react-fun/):

![screen shot 2014-05-18 at 3 28 43 pm](https://cloud.githubusercontent.com/assets/590904/3008566/b57cf6aa-dec2-11e3-9eec-4f48a2fbe987.png)

Also, I wasn't able to find tests for this script, do any exist?