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
Difficult. I have to admit that I did not think about writing the tags when I added these lines. Maybe the best solution would be to overwrite only existing tags or create an empty one if there is none of these available.