tests: Add a test for the external handlers code path
tests: Add a test for the external handlers code path

This test demonstrates how to pickle objects that are not
directly handled by jsonpickle.  The test uses mixin classes
derived from unicode.

Closes #1

Signed-off-by: David Aguilar <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>

mandx/jsonpickle,dongguangming/jsonpickle,dongguangming/jsonpickle,mandx/jsonpickle,mandx/jsonpickle,mandx/jsonpickle,dongguangming/jsonpickle,dongguangming/jsonpickle,eoghanmurray/jsonpickle_prev,eoghanmurray/jsonpickle_prev