Label handling for Neography::Node objects

Adds `set_labels`, `add_labels` and `delete_label` methods for nodes and
caches the values fetched from neo4j so that multiple calls of `labels`
only issue one request.

As with node props, the cached labels can be set from the outside.
Merge pull request #179 from jayniz/features/phase-2-node-labels

Label handling for Neography::Node objects