Add status bar style for iOS

Previously, when added to the home screen, 2048 would show up like this on iOS (notice the statusbar):
![White text against beige background](http://cl.ly/UcAF/old.png)

This PR changes it to this:
![White text against black status bar on top](http://cl.ly/Uc9j/new.png)

(Sorry about the huge images)