added byref to dill.dump for pickling by name reference where available;
added T6-group for pickling of 'special objects' (namedtuples);
added test for namedtuple to test_classdef


git-svn-id: svn+ssh://svn.mystic.cacr.caltech.edu/pathos/dill@552 8bfda07e-5b16-0410-ab1d-fd04ec2748df
Merge branch 'master' of https://github.com/uqfoundation/dill