Specializing IndexedRDD as VertexSetRDD.

1) This allows the index map to be optimized for Vids
2) This makes the code more readable
2) The Graph API can now return VertexSetRDDs from operations that produce results for vertices
Merge pull request #28 from jegonzal/VertexSetRDD

Refactoring IndexedRDD to VertexSetRDD.