Enable Titan to return multiple Vertices from list of IDs

[kfoss]

Edges contain _inV and _outV, which point at the IDs of respective vertices. Currently, each vertex must be retrieved one by one. It would be helpful to query Titan for multiple vertices, especially when working with a list of edges. For example, providing {'id1', 'id2', 'id3', ... } to return all vertices would be a more efficient query.