Also:
- Made self.locals_in_py = True for main.py and examples/python_scripting.py
    - This makes debugging easier in the embedded IPython shell provided by the opt-in ipy command

Added some shared definitions to cmd2/__init__.py -> maybe there's a better place for these?
Figured out how to trick bash into showing argument hints. It's a bit weird.
Updated all of the tests and examples to import cmd2 resources from their new location without the automatic imports in cmd2/__init__.py
For #369

Removed all dependency on six other than for six.moves.input

Also:
- Started removing code branches which were for Python 2 support

- Since path_complete got moved outside of cmd2.Cmd and is now a function instead of a method, need to do partial argument binding with functools.partial() instead of functools.partialmethod()

Removed usage of and reference to attributes and commands which have now been removed.

Also added another @with_argparser_and_list decorator that uses argparse.parse_known_args to pass two arguments to a command:  both the argparse output and a list of unknown/unmatched args.