Add a `style` property to the TweetView

[ognen]

This is a rect native style and will be used to layout the tweet view (like other react views). The style's backgroundColor should be mapped to the backgroundColor of the TWTRTweetView`.

See: https://facebook.github.io/react-native/docs/native-components-ios.html#styles

(probably not much to do except to document the style prop and to make sure it passes on to the container as well as the bg-color).

[andianton]

• done on iOS