Merging with spark upstream changes.
Merge branch 'master' of https://github.com/amplab/graphx
Added some stub code to address the case where a vertex could occur multiple times in the vertex table or where a vertex in the edge list may not appear in the vertex table.

Moving IndexedRDD into the graphx source tree and removing dependencies in /core.
Merge pull request #27 from jegonzal/removed_indexedrdd_from_core

Removing IndexedRDD changes for spark/core