Allow multiple storage backends for Node data

[niho]

The intention is to allow you to use any ActiveModel-like storage backend you want to store the Node data. Currently Related stores a node as a Redis hash using Related::Entity which is a simple ActiveModel-like object-hash-mapper. The querying of the graph is largely decoupled from the storage layer and implemented in the Related::Node::QueryMethods module and Related::Node::Query class. Adding the same graph querying ability to any other compatible class should therefore be fairly trivial.