Fetch and Streaming API

[bblfish]

I have a beginning of a Fetch and Streaming API
https://github.com/read-write-web/rww-scala-js/tree/akka.js/src/main/scala/org/scalajs/dom/experimental

As I use it and make sure it works I'd be happy to add it to the repo here. I have been trying to work on getting as much typesafety as possible out of the Fetch API spec.

I have some initial code demonstrating the working of the Fetch API. I have not yet seriously tested the streaming one yet. As it stabilises I'd be happy to contribute it back. Or else publish it seperately of the library I am building right now.