Label object and stub Client method for retrieving labels
Basic support for getting nodes matching a label
Handle some exceptional and error conditions with querying labels
Client generates/retrieves label instances

Also, fix a bug in variable cache where no expiration time mistakenly meant "expire immediately"
List labels already on the server
List labels for a specified node
Get labels for a node from the node object
Rename methods for consistency
Rename for consistency
Add labels to a node
Remove labels from a node
Capabilty testing and fail if labels are not supported
Centralized capability checking for Gremlin and Cypher