Merge pull request #247 from seantis/master

Adds the ability to pass arguments to the tox command.
Overriding predicates and fallbacks works just fine. Closes #245.
Demonstrate that additional wrong keys are ignored by Reg. Needs
MASTER Reg.
No coverage for Python 3 compatibility code.
Increase coverage for config.py by testing this scenario.
Remove now-unused code.
Simplify the exception tween factory: view.response cannot
return None anymore.