Theming support

Define a theme like: 

```
var theme = {
   ComponentDisplayName: {
      styleName: ReactStyle(function() {
           backgroundColor: 'pink'
      })
   }
};
```

Use a theme within a view (/component)
```
var SomeView = React.createClass({
   themes: [require('a theme')]
   render: function() {
      return <ComponentDisplayName />;
   }
});
```

Does not work for inline styles, as I don't see this easily supported due to possible mixing of classes and inline styles.