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
Do you know what software uses these particular `desc` fields? The reason I ask is that we *should* be able to read a COMM frame with any `desc` just using the existing style. Then, on writing back, only one frame (with no `desc`) will be present. With these multiple styles, MediaFile's "read any, write all" compatibility policy will lead to several copies of the same data, which could actually be problematic since people tend to abuse the comments field for storing all manner of (potentially large) data.