F4VDownloader: Add support for parsing DRM header and decrypting video

This is still a work in progress.  The decrypt and parser routines work OK,
but the downloader still cannot automatically retrieve the decryption key so
it currently has to be passed in by the site.  It should eventually be
possible to retrieve the key from the token.

Signed-off-by: James Bottomley <JBottomley@Parallels.com>