enable Pickler and Unpickler to be extended; added associated test


git-svn-id: svn+ssh://svn.mystic.cacr.caltech.edu/pathos/dill@612 8bfda07e-5b16-0410-ab1d-fd04ec2748df
Merge branch 'master' of https://github.com/uqfoundation/dill

Conflicts:
	tests/test_extendpickle.py