Add Kruskal algorithm MST implementation

Kruskal's algorithm computes MST in time proportional
to ElogE in worst case.

operation	frequency	time per op
*********************************************
build pq	1		ElogE
delete-min	E		logE
union		V		log*V
connected	E 		log*V
           log*V <=  5
If edges are already sorted, order of growth is Elog*V