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

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.

1) cmd2 no longer imports make_option from optparse
- test files and examples now import this directly
- this helps emphasize that this old optparse methodology of adding options to commands is deprecated

2) All argparsers have been given custom names instead of just "argparser"
- this helps with readability and maintainability, especially with IDE renaming and such