Line of sight

Add methods to GameObjectMap to find line-of-sight between objects. Specifically: #collisions_with, #each_game_object_between, #game_object_between?, #game_object_at, and #grid_spaces_between.

Here's an example of its use: 

    player.sees_enemy if game_object_map.game_object_between?(player, enemy, :only => Wall) # Walls block vision

Also another small change to GameObject#load_game_objects to specify the exception that caused the error. I have a further (commented-out) suggestion to re-raise such errors, but that is outside the scope of this branch.