jsonpickle: honor __getstate__ and __setstate__ for dict subclasses

Rework the picklers so that the __getstate__ machinery is activated on
dict subclasses.  Add elaborate tests to ensure that it interacts
correctly with the referencing machinery.

Closes #59

Reported-by: @as45239 via github.com
Signed-off-by: David Aguilar <davvid@gmail.com>