Guide to dependency injection and service lookup in Ember.js

A guide to DI and service lookup, replaces #1293
I'm not sure that `template:index` is actually a factory.  Templates are not instantiated (see [here](https://github.com/emberjs/ember.js/blob/master/packages/ember-application/lib/system/application.js#L944)), they are just functions that are re-used repeatedly.