fix detection of media url on youtube
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
gdcvault: Implement searching; partial download fix

* Implement search functionality, using POST to get JSON data
* Fix download for most items even for non-free ones.
For now only missing are non-free mp3 files it seems (like 769),
/mediaProxy.php returns 2bytes html crap.
Check if repositories are ok at the application runtime

Closes #1072