Add JSChannel

[JSChannel](http://mozilla.github.io/jschannel/docs/) is a small JavaScript abstraction layer on top of HTML5 cross-document messaging. It builds rich messaging semantics out of `window.postMessage()`.

Script source: [src/jschannel.js](https://github.com/mozilla/jschannel/blob/8c4f7ebaabe10f79e47cfaab2f19581d084bf6ac/src/jschannel.js) from [mozilla/jschannel](https://github.com/mozilla/jschannel)