fix engine counter bug & add memory graph target
merge map_reduce_vertices && retab
rewrite inf-max using  map_reduce_vertices, waiting for before_iteration api
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
beta version of filter
restore graph.hpp & ifilterinterface.hpp
Fix inf-max 2 and 3
Retab
map_reduce_edges
remove xcode project file for convenience
Fix it
Merge pull request #46 from kimiyoung/inf-max

Fix inf-max 2 and 3
Merge pull request #47 from kimiyoung/map_reduce_edges

map_reduce_edges
added context in mapreduce interface
Merge pull request #50 from neozhangthe1/map_reduce_context

added context in mapreduce interface