Fix a bug in Sum() and add all id3frames in Raw()

The Seek in SumID3v2() didn't take into account the size of the ID3v2 Header
(10 bytes). Thus when tagging the same file with or without it's tag, there was
a discrepancy. (

And I've added a default case when parsing the id3v2 tags, by storing the
binary content to Raw()

sum.go

@@ -141,7 +141,7 @@ func SumID3v2(r io.ReadSeeker) (string, error) {
 		return "", fmt.Errorf("error reading ID3v2 header: %v", err)
 	}
 
-	_, err = r.Seek(int64(h.Size), os.SEEK_SET)
+	_, err = r.Seek(int64(h.Size)+10, os.SEEK_SET)
 	if err != nil {
 		return "", fmt.Errorf("error seeking to end of ID3V2 header: %v", err)
 	}