Improved pathfinding

Currently the pathfinding used by NPCs isn’t very natural. NPCs often gets stuck in the surroundings and move unnatural (i.e. always in a straight line). This needs to be improved to make the world appear more natural. Also, as we want to further on add game rules which prevents entities from moving over too steep terrain we need to make NPCs handle this. An improved path finding algorithm along with new movement code need to be implemented. For this we should reuse existing work such as Recast/Detour and OpenSteer.