Implement isomorphic fetch

[safinsingh]

As stated in #456, this library seems to be for both the browser and Node.js; however, it uses node-fetch which is a request library specifically for Node because it uses modules not present in the browser. This PR implements an isomorphic fetch using cross-fetch so that it can properly run in the browser. It maintains the @types/node-fetch package as a dev dependency so that certain type casts still work.

[gr2m]

node-fetch should not be bundled for browser builds, a per https://github.com/node-fetch/node-fetch/blob/e218f8d5b7c6ad48b3a6c8e85bc65948ed295b26/package.json#L6. This library is usable in the browser today