Added slash separator to DateField and more comments styles to MediaFile

I have added some StorageStyles to the comments field, which I have seen appearing in actual files.

Additionally, I have changed the DateField to also allow slashed as separator. An example file where this date format appears can be found at http://commons.wikimedia.org/wiki/File:Tromboon-sample.ogg
The fetch method in MP3DescStorageStyle checks if the given description and the frame description are the same. This is correct IMO, because e.g. iTunes uses the COMM frame with different descriptions for hidden fields, see http://joelverhagen.com/blog/2010/12/how-itunes-uses-id3-tags/

For ID3v1 there is an example at https://gnunet.org/svn/Extractor/test/id3v1/id3v1_006_basic.mp3. For the others I will check, if I can find a sample file.