Wrong track duration posted to Playlog

The durationSeconds posted to playlog seems to be wrong depending on the file type. The duration of mp3 files is sent correctly, the duration of flac files is completely wrong. Other file types where not tested so far.

We use Liquidsoaps request.duration to retrieve the duration of a given audio file. This seems to be bugged - with a newer Liquidsoap version the duration of a given flac file is calculated correctly.

When #70 (closed) is implemented, this should be fixed.