Merge pull request #27 from jegonzal/removed_indexedrdd_from_core

Removing IndexedRDD changes for spark/core
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