Added workaround for blank Message Center in iOS 8 beta.
The problem is that `systemLayoutSizeFittingSize:` returns height of 0 in iOS 8 beta 6
http://openradar.appspot.com/17958382
Fixes IOS-616
Clarify `keyboardShown`
Fixes issue where Message Center textfield floats well above the keyboard in iOS 8.
I believe this is a bug with `UIModalPresentationFormSheet` and its automatic movement for keyboard in landscape.
Fixes IOS-618
Merge branch 'iOS8' into seattleDesign