Include file date and configure storage of gradebook_attachment files

Contributed by madflute at http://groups.drupal.org/node/6447#comment-67736:

I need to be able to know when student uploaded the file. This is because of the deadline. Additionally, it would be super if we can close the upload per assignment to prevent from late submission.

Another issue is that the uploaded files are stored under default file directory. It would be very much helpful if we could control this, i.e., sub directory per OG (class).