Mopub exchange video

Added video support for mopub exchange, added a log message if we get a mraid message to say we don't support the fields in there
This whole block would be more clear using local variables.
int minduration = 0;
if(video.isMember("minduration")) {
  minduration = video["minduration"].asInt();
  spot.video->minduration = minduration;
}

Then below you can just test and use minduration and avoid the redundant isMember("minduration") check and just check for minduration == 0. The code wiill be shorter and easier to understand.