add mask operation to Graph
add filter operation to GraphOps
Merge remote-tracking branch 'upstream/master' into add_project_to_graph

Conflicts:
	graph/src/main/scala/org/apache/spark/graph/Graph.scala
	graph/src/main/scala/org/apache/spark/graph/impl/GraphImpl.scala
Reimplement Graph.mask using innerJoin
Merge pull request #1 from ankurdave/add_project_to_graph

Merge current master and reimplement Graph.mask using innerJoin
Merge pull request #48 from amatsukawa/add_project_to_graph

Add mask operation on graph and filter graph primitive