Create classes

[michaelplatt07]

Current classes required:

• Sprite: top level class
• Actor(Sprite): possibly an interface. Things that have actions
• Terrain(Sprite): the ground and its textures
• Unit(Sprite): represents all heroes, enemies, pets
• Item(Sprite): equipment, consumables, anything collectable in the game.

[michaelplatt07]

Sprite completed for now. Unit completed for now. Other classes to be implemented soon.