Pullreq skip nulls on write

These commits allow us to 
1) not save nulls in a Kiji table automatically on a per column basis (this behavior is off by default)
2) have a Missing token in a cascading tuple that will never be saved in Kiji (as an alternative for those that do want to save nulls)

I also took the liberty to refactor ColumnRequest a tiny bit, introducing the case class copy method. I think for building fluent interfaces it is better to use copy than a constructor. The main reason is that as parameters (vals) get added to case classes we typically provide defaults for them in the constructor. This will mean the constructor used inside a fluent interface method will keep compiling after a parameter is added, but it will do the wrong thing (reset the new val to it's default value instead of preserving its value). Copy does the right thing.