Add option for Mailbox-style status bar "stick" to centre controller for iOS7

Hi,

See `fixStatusBarToCentreController` flag. Defaults to `NO`. Uses the snapshot technique to create the illusion that the status bar has stuck to the centre controller when any of the menus is open/panning. Tested with left/right/top/bottom menus, button open, pan open and bouncing. Seems to be nice and complete. Also added this to the demo app when running on 7.0+