This commit introduces PriorityQueue, which is a general, heap
based priority queue, and PriorityQueueItem which is an interface
that concrete priority queue items must implement.
This implementation is encapsulated, users do not need to use any
other package for full functionality.
PriorityQueue exports the usual public methids: Push, Pop, Top,
Empty and Len. For full documentaton consult the priority_queue.go,
for usage: priority_queue_test.go
