WIP: django-2 compatibility

[guglielmo]

I've created this pull request to add compatibility with django 2.

It seems that adding the correct assets to the form by forms.Media is already available in django 1.7 (https://docs.djangoproject.com/en/1.7/topics/forms/media/#media-on-forms), so I guess the patch on the media property should be back-compatible. The nasty code there is needed in order to properly merge the assets, retaining the correct order, so that jquery is loaded and initialised before the dropdown javascript.

allow_tags was deprecated in django 1.10, and substituted with mark_safe, that's taken care of checking the VERSION variable in the pull request.

I had to comment the position() method in js/jquery.dropdown.js because it moved the dropdown selector to the top-left corner of the screen.
It works in my use case, but it's marked as todo.

Please, let me know if anything is needed to merge this into your repo.