Add source map generating transform

Waiting for this https://github.com/facebook/react/pull/910

Usage:

```
grunt.initConfig({
  browserify: {
    options: {
      transform: [ require('grunt-react').browserify.withSourceMaps ],
      debug: true
    },
    app: {
      src: 'index.jsx',
      dest: 'public/index.js'
    }
  }
});
```

Transformed JSX JavaScript output has inline source map comments
appended. Browserify will then combine all these source maps when
bundling.