2015-11-26 Delay initialization of constant variable in Swift

Recent versions of Swift allow making a variable constant (with the let keyword), without directly initializing it.

This makes the following code possible:

	let identifier: String
	if flight == self.nextFlight {
		identifier = RosterListActivityCellReuseIdentifier.NextFlight.rawValue
	} else {
		identifier = RosterListActivityCellReuseIdentifier.Flight.rawValue
	}

Nice!