Y.soon speed up using MessageChannel

MessageChannel is a safe version of `postMessage` designed to communicate two objects, optionally across frames. Instead of firing a global `message` event, each MessageChannel instace has a `port1` and `port2` objects that can communicate with each other so that you can hand one of them to a foreign party.

All this makes MC an unobtrusive way of introducing a speed up in Y.soon and therefore in promises. This is what the Flickr team did in order to reduce their rendering time from 300ms to less 30-40ms or something like that using [asap.js](https://github.com/kriskowal/asap) (what @pdokas showcased during YUIConf).

The plan is to keep all the hacky versions in a Gallery module. @solmsted and I are maintaining a new repository: https://github.com/solmsted/gallery-soon.

Let me know what you think.

@rgrove you had some reservations about this...