Support ES6 default named exports for Backbone-ES6

[typhonrt]

This subtle change takes into account how ES6 modules export default named modules and how they are consumed in the UMD loading code of Backbone.localStorage. backbone-es6 (https://github.com/typhonjs/backbone-es6) is a fork of Backbone using ES6. The ES6 module output export default backbone; found in ModuleRuntime.js (https://github.com/typhonjs/backbone-es6/blob/master/src/ModuleRuntime.js#L39) puts the actual reference under a default named key in the ES5 UMD loading code.

A complete TODOs demo using Backbone-ES6 and Backbone.localStorage can be found here:
http://js.demos.typhonrt.org/backbone-es6-localstorage-todos/

and repo of the demo:
https://github.com/typhonjs-demos/backbone-es6-localstorage-todos

The change in this PR adds a check for the existence of Backbone.default in the resolution of calling factory in the Backbone.localStorage UMD wrapper.

[scott-w]

Hi @typhonrt, thanks for this, however I'm going to close it in favour of #207

[typhonrt]

Oh no worries.. Sounds good. At the time I didn't know what I do now. Back then I just rewrote local storage in ES6; I see #207 does this... Now I just use babel-plugin-add-module-exports with my ES6 modules especially everything released on NPM.