Multiple SketchField components
Hi! Is it possible to support multiple SketchField components?
Currently, it seems like the second one does not get initialized.

``` js
class SketchFieldDemo extends React.Component {
     render() {
        return (
            <div>
                 <SketchField width='1024px' 
                              height='768px' 
                              tool={Tools.Pencil} 
                              color='black'
                              lineWidth={3}/>
                 <SketchField width='1024px' 
                              height='768px' 
                              tool={Tools.Pencil} 
                              color='black'
                              lineWidth={3}/>
            </div>
        )
     }
}
```
