Added include_py keyword parameter to cmd2.Cmd.__init__(). If False, then the py command will not be available.

Removed ability to run Python commands from the command line with py.

Made banners and exit messages of Python and IPython consistent.

Changed utils.is_text_file() to raise OSError if file cannot be read.

This more accurately reflects what it controls

Issues were two fold:
- HistoryItem statement.raw was getting mangled for multiline commands due to macro-related changes in _input_line_to_statement()
- HistoryItem pretty printing wasn't using rstrip() anymore in verbose mode

I added a couple unit tests in the process of getting here.  But we should add some explicit unit tests of _input_line_to_statement() for cases like:
- basic single-line command
- macro single-line command
- multiline command

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