small map/tile changes

Written some documentation, also fixed the solid tiles not colliding
properly.

I've realised at this point that in some places, tiles will need to go
on top of one another - for example, a slope tile (half transparent,
half solid) will need a background tile underneath it. As such a matrix
of one tile per square won't cut it. I will therefore add:

a way of searching for all tiles found at x,y
a way of adding tiles to the map