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
I'm fairly sure that Mutagen is case-insensitive for tall the formats where this matters. Do you have a sample file where this doesn't work?