Stream missing videos from an online source

This addresses #695, and solves one major issue for getting KA Lite online.

Sometimes, distributed servers may be online, but if a video is not local, the students aren't able to access videos.  In some cases, this is desirable--if Internet access is limited or expensive.  But in some cases, this is undesirable--this would just cause hosting headaches.

This code allows administrators to specify a function that, given a video ID and format, computes URLs backup, online sources for videos.  The canonical backup source should be:
`    BACKUP_VIDEO_SOURCE = lambda vfile, fmt: "http://s3.amazonaws.com/KA-youtube-converted/%s.%s/%s.%s" % (vfile, fmt, vfile, fmt)`

By default, this feature is off--no backup video source. 

This should allow us to put KA Lite "online" without incurring hosting charges.