Video ads v1

This branch creates a new route /video/ad/:format which is a proxy for the OAS video advert server to parse their XML response into a nice JSON response that our new video advert javascript module can handle.

The JS module, swaps out the ```<video>``` elements source for the advert's, then plays the advert - firing off tracking events - then swaps the source back to the correct one.

This currently only supports mp4 video formats and the VAST xml schema. New video formats and schemas will be added in the future.

@gklopper can you please review my contoller code.