Fetch API

[justincy]

The fetch API defines the global fetch() method for getting resources (modern replacement of XMLHttpRequest which the sdk currently uses internally). Due to browser adoption and usage we won't be able to use it internally any time soon without a polyfill which itself depends on a polyfill for promises. Too much bloat.

But the fetch spec also defines Request, Response, and Headers objects. The sdk has it's own internal representation of those concepts. It might be useful to implement them according to the fetch spec. Though this would be a breaking change since responses are exposed to the developer.