Video js vpaid support

Here's a tentative attempt to support vpaid preoll adverts. Unfortunately it was necessary to make most of the changes in several parts of the underlying video js library. Work is based on:

https://github.com/guardian/video.js (tag v4.9.0)
https://github.com/guardian/video-js-vpaid (tag v4.5.1)
https://github.com/guardian/videojs-vast-plugin (tag 0.9.1)
https://github.com/guardian/videojs-contrib-ads (tag v.0.4.0)
https://github.com/guardian/vast-client-js (tag 1.3.1)