Add http-client for fetch

[kierans]

Feature Summary

Add an implementation of HttpClient that uses the fetch API

Feature Rationale

For browser applications, a third party client library like axios won't be required.

[kierans]

Fetch polyfills:

• https://github.com/github/fetch
• https://github.com/node-fetch/node-fetch