Add optional node label and optional node renumbering support

Add optional node label attribute support for both shared array and non-shared array structures, via new LabeledVertexReader.
Add optional node renumbering support via new SequentialNodeRenumberer.
AdjacencyListGraphReader now takes a VertexReader and NodeRenumberer, defaulting to SimpleVertexReader and IdentityNodeRenumberer which preserve existing unlabeled and unrenumbered behavior.
Add tests for both label reading and renumbering, when graph reading is done either serially or in parallel.
Passes Travis-CI tests: https://travis-ci.org/dfuhry/cassovary/builds/13414241