Make statusbar style adjustable

[fruitcoder]

Since we already have the ability to determine whether the status bar is shown or not, I think it makes sense to also expose the statusbar style through the API.
You would just have one more property statusBarStyle that the client can set before presenting the view controller. This value would be returned in preferredStatusBarStyle. I would also default the statusBarStyle to .LightContent since the dark background has better contrast for a white font.