Expose fetchBackend as an export

[derekgreer]

The restful.js library publishes two versions: dist/restful.js and dist/restul.standalone.js (although it's quite confusing that build/restful.standalone.js creates dist/restful.js and build.restful.fetch.js creates dist/restful.standalone.js).

The restful.js file exports the straight restful factory function which requires an httpBackend. The problem is that build/restful.standalone.js doesn't export fetchBackend and requestBackend. Therefore, it doesn't seem the distribution files actually support making a call like the one from the docs:

const api = restful('http://api.example.com', fetchBackend(fetch));

I've tested this with RequireJS and there doesn't seem to be a way to actually get to the fetchBackend() function.

define('restful', ['./Scripts/restful/restful..js'], function (export) {
     // export is equal to function(baseUrl, httpBackend)
    // there is no export.fetchBackend to expose to AMD modules using require
    return restful;
});

Perhaps this is by design, but because of #96 I'd like to be able to pass an instance of httpBackend with a patched version of fetch so that I can create my own interceptors.