refactored DFS/BFS to be iterators.
refactored DFS/BFS to be iterators.

tjhunter/scalanlp-core,sheide/breeze,cilier/breeze,mredaelli/breeze,ContentSquare/breeze,claydonkey/breeze,ktakagaki/breeze,calippo/breeze,ktakagaki/breeze,scalanlp/breeze,chen0031/breeze,crealytics/breeze,tjhunter/scalanlp-core,eponvert/breeze,wstcpyt/breeze,nikdon/breeze