PICARD-42: Support AIFF files

This adds support for AIFF files, which can be tagged using ID3 (see [PICARD-42](http://tickets.musicbrainz.org/browse/PICARD-42)). mutagen 1.23 is required for this to work, I actually tested and developed it using mutagen 1.25.

mutagen's ID3 implementation for AIFF differs in some respects from the ones for MP3 and TrueAudio, but I have tried to keep most of the code unchanged.

For testing I have used files from http://www-mmsp.ece.mcgill.ca/documents/AudioFormats/AIFF/Samples.html and the `test.aiff` included in the pull request which I encoded from the `test.flac` file:

```
flac -d --force-aiff-format --output-name=test.aiff test.flac
```

Side question: Am I supposed to update the NEWS.txt or will we handle this all later via http://tickets.musicbrainz.org ?