algebra.Shuffle: make hash_columns a list of AttributeRefs

and fixup the tests

See #203
StringLiteral: print with quotes

See #203
raco.viz: fix escaping labels on nodes
implement __ne__ with __eq__

according to https://docs.python.org/2/reference/datamodel.html#object.__ge__

...'when defining __eq__(), one should also define __ne__()'...
Merge pull request #206 from uwescience/fix-203

Fix 203