Basic light system

Added lighting using the sky as a source of light. Probably need to
re-write the whole thing if I want to add other light sources.
More buggy lighting

Added lighting calculations based on neighboring blocks.
Added light block, doesn't work

Added a light block, now to make it work.
Improved lighting

Improved lighting, still need to make lighting effect the blocks above
the light block.
Removed some comments
Fixed crashes
Fixed block "shadows" and another crash
Moved some commands around

Moved canSeeSky(x,y) to the main class to be used as a static command.
Also renamed Block.canSeeSky() to avoid naming conflicts.
Fixed line of sight code

Found an issue with how the blocks calculated whether or not they could
see the sky, so I re-wrote that code.
Better lighting

Re-wrote lighting again. This time using a lightmap.
Merge pull request #10 from carterv/alternate-lighting-test

Added Lighting Engine.