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.

# Conflicts:
#	cmd2/__init__.py
#	cmd2/argparse_completer.py
#	cmd2/argparse_custom.py
#	cmd2/cmd2.py
#	cmd2/decorators.py
#	cmd2/exceptions.py
#	cmd2/utils.py
#	examples/arg_decorators.py
#	examples/argparse_completion.py
#	examples/modular_commands_main.py
#	tests/test_argparse_completer.py
#	tests/test_argparse_custom.py
#	tests/test_cmd2.py
#	tests/test_completion.py
#	tests/test_history.py