Minify both before and after browserify

Excludes `ReactDefaultPerf` and `performanceNow` from the minified build, so we save bytes. Looks like there are a few other minor differences because uglifyjs isn't quite idempotent but they appear to be harmless.

Feels silly but I don't think there's another way. (We need to run uglify after as well so that require, module, exports are minified)

```
   raw     gz Compared to master @ f877c6224fbf217d639a0ef2d7abf28c4cfbafa4
     =      = build/JSXTransformer.js
     =      = build/react-test.js
     =      = build/react-with-addons.js
  -406   -129 build/react-with-addons.min.js
     =      = build/react.js
  -406   -108 build/react.min.js
```