Fix changelog generator script
    
     * Correctly exclude merge commits (there's a git log option for it)
     * Stop mangling log lines with @ symbol. This should only be cut for
       author of a change.
     * Fix PEP8 issues
     * Remove unused code
     * Port to argparse (optparse is deprecated)
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>