Bug 830497 - fix ID3v2.2 metadata parsing

This pull request fixes a stupid error in MP3 metadata parsing. If an mp3 file had ID3v2.2 metadata, we ignored the album metadata and used whatever value the previous tag had.

This fixes that, and also changes the way we release <img> and <audio> resources by calling removeAttribute('src') instead of src=null.  The latter technique causes extra warnings in the log and also (for images) has been known to cause infinite loops of the onerror callback.