Target filename for transcoding (ticket #134)

Adresses ticket #134.
Without this the script outputs files without audio content.
Correct 'COMM' ID3v2 frame (ticket #94)

Addresses ticket #94.

Correct form of the 'COMM' ID3v2 tag. Without this, many tagging
programs (including MP3Tag) will not properly recognise the comment tag
(displaying an empty comment tag).

In the old version 'Short Text' was hard coded into the descriptor
field. This field should be empty.