Return Graph from aggregateNeighbors; update callers

This commit only affects the Graph API, not GraphImpl.
Implement aggregateNeighbors returning Graph
Return Graph from default aggregateNeighbors also
Merge branch 'master' into aggregateNeighbors-returns-graph
Add a unit test for aggregateNeighbors
Merge pull request #1 from ankurdave/aggregateNeighbors-returns-graph

Return Graph from Graph.aggregateNeighbors