Implemented Overrun

Implemented overrun for experiments.
HDA* continue searching until expanding all nodes with f value less than
optimal solution cost + overrun, whereas overrun is given by a command
line option -o-%d.
Auto commit 120521
Implemented Zobrsit Hashing for HDA*

Zobrist hashing improves HDA* load balancing and thus # of node
expansions.
The quality of hash function is key to its performance in HDA*.