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
Got it; thanks for clarifying.

It seems like the appropriate strategy here (which we don't currently implement) would be to read a frame with *any* of these `desc`s, but when writing, don't create one that doesn't already exist. Or, perhaps, always write a COMM frame with an empty `desc` for comments and drop other COMM frames with those specific `desc`s. We could specify either by making the parameter to `MP3DescStorageStyle` a list of strings rather than a single string.

Any opinions either way?