Modulation of valueMap()

Allow for modulation of the valueMap() step with by() and possibly with() to make it's usage more flexible and consistent. The problem presented in relation to this change is how to easily unwrap the general multi-property structure of valueMap()

g.V().has('person','name','marko').
  valueMap('name','age').
    by(unfold())

which would yield: [name:marko,age:29] rather than: [name:[marko],age:[29]]. Obviously the by() is just a map over the values in the list of values returned from properties.

Please see the DISCUSS thread for additional thoughts on the matter to consider.