Adding a briefing screen object, a briefing game state, and modified constructors for existing objects

Work was done to allow for mission briefings and debriefings.  When you
change the game state to BRIEFING you pass in a map name, and up to 3
strings that serve as the briefing text.  When you press enter from the
briefing screen the specified map is loaded into a PlayScreen object
and you can play a level.  When the ship's posiiton equals 0 the
GameOver screen is loaded.