Update units.py

Changed WithUnit so it's no longer a mixin. Value does not subclass float, etc.
Update test_units.py

updated to reflect new WithUnit
Merge pull request #1 from pomalley/master

WithUnit no longer a mixin class.

Having Value subclass both WithUnit and float caused lots of unexpected behavior so we have removed the subclassing of float and converted WithUnit to a straight supeclass instead of a mixin. Similar for Complex.