Minor changes, Experimenting with different ways of handling events and
messages in regards to data flow, update  schedule, and especially game
logic. 

Added tentative asset management system.

Added system that manages grid operations. 
MovementSystem handles movement logic (sliding on ice, pushing)
GridSystem handles grid operations (changing entity location,
determining whether entities should be removed (on a hole). If moving
off of a cracked tile, changing tile to a hole. Perhaps I should combine
both systems since the distinction between the two is somewhat blurred
(especially when movement or state in the grid is dependent on tile
state.