Add map for which tiles have been "seen"

[paralin]

... to differentiate between a unseen plain tile and a seen, known to be empty, plain tile

[harrischristiansen]

The Map class now has built in memory. To check if a tile has been seen before or not, try:

TILE_EMPTY = -1
map.grid[y][x].tile >= TILE_EMPTY.

Tile values >= 0 are players, -1 is empty, -2 is mountain, and lower is unseen.