Pathfinding grid improved.

There is now automatic calculation of passable and non passable
pathfinding node no matter what density the grid is set too, or whatever
size the map is at, just need to make sure you set the correct tiles to
Passable/non-passable. The pathfinding grid has been restructured so
every node has its own class for easy addition of values for the
pathfinding calculations(im thinking the A* coefficients.)

The most relevent functions are probably SetTile() and
RecalculatePathfindingGrid() Remember SetTile & the tile class have to
be maintained when adding a new tile type.