Allow usage of "className"

Hi, I use tachyons [0] to style react for web, so I'd like to use the `className` property in react-native as well.

It allows me to use "mini"-css classes and write code like this, instead of using the `styles` property, which I find way better (after having tried both approaches).

``` javascript
   <View className="w1 wv">

   </View>
```

[0] http://tachyons.io/docs/
