Release 4 -- Depth Update

This update added stairs that lead to additional floors.

• Added stairs
• Added a "final floor" that also spawns a boss monster. Currently that is floor 2
• Tiles containing more than one item, or an entity and an item, are highlighted
• Pressing 'm' shows the message log, and is scrollable

Other miscellaneous changes:

• Menus are now closed after loading a game
• HP is set to 500... made finding the boss monster a little easier!