Merge pull request #55 from ankurdave/aggregateNeighbors-variants

Specialize mapReduceTriplets for accessing subsets of vertex attributes
Merge pull request #61 from ankurdave/pid2vid

Shuffle replicated vertex attributes efficiently in columnar format
Cleanning up documentation of VertexSetRDD.scala
Merge pull request #63 from jegonzal/VertexSetCleanup

Cleanup of VertexSetRDD
Update README.md

Changing image references to master branch.
Fixed issue with canonical edge partitioner.
Added PartitionStrategy option
Updated documentation