Guide to dependency injection and service lookup in Ember.js

A guide to DI and service lookup, replaces #1293
Right. But we need a word for "what is in the container, and you manage dependencies of". IMO the template's role in the container is most easily described as a singleton factory that does not instantiate. I know it bends the rules of what a factory is, because "do not instantiate" is an odd concept for a factory.

I'm open to changing this if we have better terminology, but factory is the internal language right now. For instance, `lookupFactory` - still returns a template.