allow_cli_args is now an argument to __init__ instead of a cmd2 class member

cmd2.Cmd.cmdloop() now returns self.exit_code which should be an integer

Also:
- Refactored examples to call sys.exit(app.cmdloop()) in their __main__
- Running transcript tests now sets the exit_code accordingly based on success/failure
- Updated CHANGELOG
- Updated README
- Updated Sphinx docs
- Added unit test for case when transcript test fails

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

Added pexpect to modules required for running unit tests.

This opens the door for carefully crafted complex unit tests to verify intricate behavior.  Tests like this are somewhat painful to write and slow to execute.  However, they can enable testing complicated interactive behavior that we otherwise probably would not be able to test.

- Including an example and info in the Sphinx docs

Also:
- Created CHANGELOG entry for 0.8.1 release
- Added info to README about new sub-menu feature
- Bumped version to 0.8.1

TODO:
- Added a unit test for the persistent readline history feature