Remove non-relative Seek calls from all functions.

Previously we would seek to the beginning of the io.ReadSeeker. It is much more
flexible to assume that the caller passes the ReadSeeker in an appropriate state.

ogg.go

@@ -20,11 +20,6 @@ const (
 // See http://www.xiph.org/vorbis/doc/Vorbis_I_spec.html
 // and http://www.xiph.org/ogg/doc/framing.html for details.
 func ReadOGGTags(r io.ReadSeeker) (Metadata, error) {
-	_, err := r.Seek(0, os.SEEK_SET)
-	if err != nil {
-		return nil, err
-	}
-
 	oggs, err := readString(r, 4)
 	if err != nil {
 		return nil, err