Add a configuration option for enabling the JSX Harmony compile

I wanted a way to use the Harmony command line switch automatically from Rails. I've added it as a simple option:

```
MyApp::Application.configure do
  config.react.harmony = true
end
```

I also included a simple test case. 
I don't know if we ever made any formal declaration that we don't support Ruby 1.8. I think all of our other hash uses continue with the rocket syntax.

Additionally, we should just make `harmony` an option to the transform function (or better, take an options hash) instead of relying on a global config like this.