check zero length of playlists
added option to use external transcoder

use settings.transcode to define which external process use to reencode
files, e.g.

transcoder = "/opt/local/bin/lame --resample 44100 --mp3input --vbr-new
-b 112 -B 128 -m j -q 2 - -"

external process have to accept stdin and stdout as input/output
sources.