'print_version(file : file = stdout)
Print the version message for this program (self.version) to
\'file\' (default stdout).  As with print_usage(), any occurrence
of "%prog" in self.version is replaced by the current program\'s
name.  Does nothing if self.version is empty or undefined.'

def print_version(self, file=None):

    if self.version:
        print >>file, self.get_version()
