swc-installation-test-2.py: Add --help and --verbose

The system information is useful for troubleshooting a student's
installation problems, but it is probably less useful for the student
themselves.  Turn off this troubleshooting information by default and
require the --verbose option to re-enable it.

If we didn't have to maintain support for Python 2.6, we could use
argparse instead of optparse.  The optparse module is deprecated for
2.7, 3.2, and later, but it is still part of the stdandard library.

Also update the README to point out --verbose.