Added support for unpack(To) -> '* as per:

https://groups.google.com/forum/?hl=en&fromgroups=#!topic/cascading-user/49OkfPAotx8

Involved updating ReflectionSetter's arity() method and adding a
getResultFields() method to TupleUnpacker (and overriding the default
implementation in ReflectionTupleUnpacker). Without these, Fields.ALL would
cause problems for Cascading downstream.