StatusBar fixes for iOS6 and 7

Messages are now displayed underneath the StatusBar in iOS7.

I've also added the method ```-updateMessageFrames``` to ```TWMessageBarManager```. This should be used manually whenever the statusbar hidden status is changed. It will change the frame of any currently visible ```TWMessageView``` to match the statusbar appearance. As I've noted in the comment for the method this should ideally be called automatically, but without any swizzling we won't get any notifications of any statusbar appearance changes. Check the demo for an example of how to use this method.

I've also added a "Toggle StatusBar" button in the demo application to test this behavior. Toggle the statusbar while a message is displayed to test my added functionality.

Tested in iOS7 and iOS6