Make video debug messages consistent.

In the JavaScript console all messages from the Video
player should begin with "[Video info]: ". This way it
is easier to see which console.log() messages are from
the Video player, and which console.log() messages are
temporary, and should be removed before a merge/commit.
Merge pull request #2272 from edx/valera/make_video_debug_messages_consistent

Make video debug messages consistent.