wip delta join.
In static Pregel, replicate only changed vertices
Test Graph.deltaJoinVertices and VertexRDD.diff
Revert changes to project/SparkBuild.scala

Changes were made in 3fc4534d19a7abf7dcb6c9ebe17e5e29f1febbc5
Re-roll last iteration of static Pregel loop
Use mask for dynamic Pregel
Merge branch 'pregel-replicate-changed' of github.com:ankurdave/graphx into pregel-replicate-changed
Test prevVTableReplicated code path
Simplify GraphImpl.deltaJoinVertices
Merge branch 'pregel-replicate-changed' of github.com:ankurdave/graphx into pregel-replicate-changed
Fixed a bug in VTableReplicated that we are always broadcasting all the vertices.
Minor update.
Fixed a bug in VTableReplicated that we only process the first block.
Merge pull request #90 from amplab/pregel-replicate-changed

Replicate only changed vertices