Properties on Elements

Problem: When a user writes `g.V()` they get back a Vertex object. The problem is that depending on the execution context of the traversal, the result could be quite different, with or without properties.

Solution: Implement new finalization strategy DetachStrategy(detachMode, properties) where mode is one of ALL, NONE or CUSTOM. `properties` is list of properties name, are taken into account only for CUSTOM mode.

Discussion thread in dev list: Proposal to handle properties on response Elements-Apache Mail Archives
Stephen's thread in dev list: The Issue of Detachment-Apache Mail Archives