Also fix bug caused by _properties being a class variable rather than an
instance variable. If you created an object with allow_custom,
allow_custom would be set for all future instances.
Import a bunch of stuff so users can just "from stix2.workbench import *" and
not need to import other stuff (e.g. MarkingDefinition, Cyber Observable
Object classes, etc.) from stix2.
This is needed to implement functions like `set_default_creator`.
The changes to Tox are so that the wrapping we do in workbench doesn't
affect the rest of our tests. If we test them all in one go, pytest will
import all the tests before running any of them. This will cause the
workbench versions of the SDO classes to be used in all tests.
Chris Lenk