Update html5.js

These changes enable ads with video content running over the flowplayer player on Android and iOS devices. The logic behind this code is this:
- on init, if the plugin needs to display some video content (for example an ad), it sets the allowVideoOvertake variable to true.
flowplayer(function(api, root) {
    api.allowVideoOvertake = true;
    ....
}

- The plugin must then listen for the overtakeVideo event to be fired, and the video(s) from the plugin must be loaded and initialized at this point.