Add sourceMap option to react-tools transform

Allow tools like grunt-react to include inline source maps in the
generated JavaScript. Browserify can then combine these source maps when
bundling everything together.

Usage:

```
var transform = require('react-tools').transform;
var output = transform(jsxContent, {
  sourceMap: true,
  sourceFilename: 'source.jsx'
});
```

The `output` will have an inline source map comment appended.