account for non-ascii filenames

s3cmd was crasing with the following error message, when trying to sync a file whose filename has accented (non-ascii) characters:

    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
        An unexpected error has occurred.
      Please report the following lines to:
       s3tools-bugs@lists.sourceforge.net
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    
    Problem: UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 123: ordinal not in range(128)
    S3cmd:   1.1.0-beta3
    
    Traceback (most recent call last):
      File "/usr/bin/s3cmd", line 1800, in <module>
        main()
      File "/usr/bin/s3cmd", line 1741, in main
        cmd_func(args)
      File "/usr/bin/s3cmd", line 967, in cmd_sync
        return cmd_sync_remote2local(args)
      File "/usr/bin/s3cmd", line 768, in cmd_sync_remote2local
        warning(u"%s is a directory - skipping over" % dst_file)
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 123: ordinal not in range(128)
    
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
        An unexpected error has occurred.
        Please report the above lines to:
       s3tools-bugs@lists.sourceforge.net
    !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Performing the change suggested in this pull request fixed the problem.