Watch directory for changes and auto pack sources

def watch(args):
    " Watch directory for changes and auto pack sources "
    assert op.isdir(args.source), "Watch mode allowed only for directories."
    print 'Zeta-library v. %s watch mode' % VERSION
    print '================================'
    print 'Ctrl+C for exit\n'
    observer = Observer()
    handler = ZetaTrick(args=args)
    observer.schedule(handler, args.source, recursive=True)
    observer.start()
    try:
        while True:
            time.sleep(1)
    except KeyboardInterrupt:
        observer.stop()
        print "\nWatch mode stoped."
    observer.join()