gdcvault: implement all 4 media types

* We now retrieve correctly all 4 media types:

** Video (plain flv file)
ex: http://gdcvault.com/play/29

** Audio (mp3)
ex: http://gdcvault.com/play/22/From-One-off-to-Franchise

** Slides (pdf, not really a video but they download fine)
ex: http://gdcvault.com/play/1015486

** DS Video (dual screen, two flv files) (by default the speaker file,
but id#slides gets the other video (however default filenames conflict)
ex: http://gdcvault.com/play/1015841