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.

setup/README.md

@@ -21,11 +21,29 @@ If you see something like:
     $ python swc-installation-test-2.py
     check virtual-shell...  fail
     …
-    For instructings on installing a particular package,
-    see the failure message for that package printed above.
+    check for command line shell (virtual-shell) failed:
+      command line shell (virtual-shell) requires at least one of the following dependencies
+      For instructions on installing an up-to-date version, see
+      http://software-carpentry.org/setup/
+      causes:
+      check for Bourne Again Shell (bash) failed:
+        could not find 'bash' executable for Bourne Again Shell (bash)
+        For instructions on installing an up-to-date version, see
+        http://software-carpentry.org/setup/
     …
 
-follow the suggestions to try and install any missing software.
+follow the suggestions to try and install any missing software.  For
+additional troubleshooting information, you can use the `--verbose`
+option:
+
+    $ python swc-installation-test-2.py --verbose
+    check virtual-shell...  fail
+    …
+    ==================
+    System information
+    ==================
+    os.name            : posix
+    …
 
 Instructors
 ===========

setup/swc-installation-test-2.py

@@ -799,8 +799,18 @@ def print_suggestions(instructor_fallback=True):
 
 
 if __name__ == '__main__':
+    import optparse as _optparse
+
+    parser = _optparse.OptionParser(usage='%prog [options] [check...]')
+    epilog = __doc__
+    parser.format_epilog = lambda formatter: '\n' + epilog
+    parser.add_option(
+        '-v', '--verbose', action='store_true',
+        help=('print additional information to help troubleshoot '
+              'installation issues'))
+    options,args = parser.parse_args()
     try:
-        passed = check(_sys.argv[1:])
+        passed = check(args)
     except InvalidCheck as e:
         print("I don't know how to check for {0!r}".format(e.check))
         print('I do know how to check for:')
@@ -812,7 +822,8 @@ def print_suggestions(instructor_fallback=True):
                 print('  {0}'.format(key))
         _sys.exit(1)
     if not passed:
-        print()
-        print_system_info()
-        print_suggestions(instructor_fallback=True)
+        if options.verbose:
+            print()
+            print_system_info()
+            print_suggestions(instructor_fallback=True)
         _sys.exit(1)