fix engine counter bug & add memory graph target
merge map_reduce_vertices && retab
fix shortest_path, need new signal interface
implement signal with message, fix a bug in sync_engine constructor
merge from wei/signal_enhance
fix shortest_path
Merge pull request #37 from pondering/signal-enhance

implement signal with message, fix a bug in sync_engine constructor
Merge pull request #36 from thinxer/shortest_path_fix

fix shortest_path, need new init interface
merged upstream