Allow reuse of Application.buildContainer without full Application.

This will allow usage of `Ember.Application.buildContainer` without needing
providing a namespace (or an Ember.Application instance).

The primary impetus of this is to prevent from having to create one-off 
container look-alikes for unit testing. This also allows us to consolidate the
knowledge of what things are registered and setup by default to a single
place.

This will be directly usable from within EAK (see [isolatedContainer](https://github.com/stefanpenner/ember-app-kit/blob/master/tests/helpers/isolated_container.js#L4-L9)) for example.