Bug 971618 - Add support for OMA DRM direct downloads

I didn't fix all the lint errors on download.js on this patch cause it would dillute the changes. Can do that on a continuation bug.
I think you should check for a non-emtpy cdHeader before calling getFileNameFromHeader.

Also, in the case where we actually have a .dd file with a name in it, I wonder if we should allow this header to override the name declared in the .dd file. It is probably not a big deal, however.