center randomly selected video
handle case where no videos returned for a playlist

(e.g. http://www.khanacademy.org/api/v1/topic/area--perimeter--and-volume/videos)
Removed randomized algorithms from categories

The topic url
"http://www.khanacademy.org/api/v1/topic/random%20algorithms,%20probability"
is broken.
add about modal