Added namespace cascade

Cascade through fallback namespaces when a module isn't found. Allows properly transpiled vendored packages to be used as fallbacks for a module, but still allows the app itself to override (a.k.a. shadow) those vendored modules.

For example, let's say I install ember-devise, a hypothetical auth package, it provides a LoginController. If I just run `bower install --save ember-devise`, and my build process picks up the package appropriately, then the resolver will use the ember-devise LoginController. But I can always define my own LoginController, which would be picked up instead. I could even require the ember-devise LoginController from within my version for my own use.