handlers: add unregister() function

Signed-off-by: David Aguilar <davvid@gmail.com>
Merge pull request #88 from marcintustin/master

Address roundtripping concerns [1] and [4] in issue #87.

Signed-off-by: David Aguilar <davvid@gmail.com>
Add test to ensure we handle 0j correctly
Merge branch 'master' of https://github.com/jsonpickle/jsonpickle