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.

Also updated the `Guardfile` so that unit/integration are run on a per file level from `lib/neography`.

Rspec depdency updated to `>=3.0` because the suite didn't run locally with rspec 2.x.