[RFC] New ReactDefaultPerf

This isn't the best code I ever wrote but it's better than what's there. This rewrites `ReactDefaultPerf` to be actionable and is structured so we can integrate it into the devtools later.

I did a video demo here: http://www.youtube.com/watch?v=Xc53eUYDt6I&feature=youtu.be

We could strip out the measure() calls with a build step similar to invariant() if we want to get rid of the size increase:
```
   raw     gz Compared to last run
     =      = build/JSXTransformer-previous.js
     =      = build/JSXTransformer.js
     =      = build/react-previous.min.js
     =      = build/react-test.js
 -4415  -1208 build/react-with-addons.js
  +606   +122 build/react-with-addons.min.js
-13478  -3216 build/react.js
  +606   +123 build/react.min.js
```