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.
@matthayes I would prefer to throw some sort of exception if the field names are the same, namespacing fields takes away some of the convenience of zip bags. Or maybe only namespace if the fields are actually the same and leave them as is otherwise