Added UDF ZipBags which can zip and arbitrary number of bags into one

Does this function seem useful for Datafu-pig, also because the docs are quite adamant about the ordering not being Guaranteed on DataBags is it even guaranteed to work. If it is useful I can implement unit tests for it as well.
It might be worth checking hasNext and throwing an exception if it is false.  This will help users understand the problem when they screw up.