Refined the implementation of the AStar algorithm :

*Reorganised and commented used procs for better visibility/maintenance
*Redid the PriorityQueue class
*Changed the euclidian distance to the byond one (so that we don't calculate float and square root when it's not necessary)
*Made the AStar algorithm use the closed list instead of just filling it
*Some optimization in the open list sorting : last entered is first in case of f ties, slightly less tiles are checked now
*Changes the order directions are checked when adding adjacents turfs to make for more 'realistic' path (read less 'drunk pathing' with lots of diagonals)
*Fixed some turfs densities not being check for diagonal movement
*Closed firedoors are now correctly seens as blocked by the algorithm
*Cleanbots now only moves in cardinal directions, like other bots.
*Updated the info text at the beginning of the file a bit
*Absolutely unrelated : simplified the turf.Bless() proc
Merge pull request #828 from Menshin/LetsBeAStar_NT_Edition

Dedust an ancient dark ritual : A*