Prints help text about available commands.

def show_help(self):
        """
        Prints help text about available commands.
        """
        output = [
            'Usage %s subcommand [options] [args]' % self.prog_name,
            '',
            'Available commands:',
            '',
        ]
        for cmd in self.get_commands():
            output.append('  %s' % cmd)
        output += ['', '']
        self.stdout.write(u'\n'.join(output))