Add Node#set_properties and Relationship#set_properties

I wanted to update many node/relationship properties at once (i.e. in one http request). Would this be a feasible way, or did I overlook that this functionality exists already?

This adds a `set_properties` instance method to `Property` (so it works on `Node` and `Relationship`) that updates the underlying openstruct, removes/adds getters and setters and then updates neo4j via the existing 'Rest#reset_properties`. It follows the same notion of "submit nil to delete key" logic as `[]=` does.