Improved traversers

Added abstract class QueueBasedTraverser holds general logic for most
traversers.
Fixed errors in InfoKeepers.
Added InfoKeeper that uses IntToBoolean fastutil map to store Boolean
values.
Changed BreadthFirstTraverser so that first node is the node we start
traverse from (to be consistent with other traversals).
this seems a bit too many traits to me. Can we: 
1. Define only those traits needed right now?
2. Keep BFS and DFS traversers almost exactly identical (with the exception of course of queueing priority)