Don't (or optionally) auto-inject the services

[pauln]

With code splitting on the Ember horizon (and given the size of this project's dependencies), it'd be nice to be able to turn off auto-injection and instead manually inject it where (and only where) it's needed.

Personally, I'd lean towards completely dropping the auto-injection, giving consuming apps the full flexibility to manage auto-injection however they want to; perhaps the current initializers could be added as code snippets in the readme as a starting point for anyone who wants to implement auto-loading (or simply return to the current behaviour).