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

Also:
- Bumped version to 0.8.4

Also:
- Reanamed foo and bar subcommand methods to base_foo and base_bar

I eliminated a few "narg=1" configurations so that a single str value is returned instead of a List[str].

I also reworded some documentation which was no longer correct after the last commit which made "history command" have the same help text as "command -h" when using one of the two argparse decorators.

Now "help command_name" and "command_name -h" provide exactly the same text.

The function docstring for the "do_*" command sets and overrides the ArgumentParser "description" if the docstring is not empty.

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.