[WIP] docs: add DI cookbook

[thelgevold]

ATTENTION: this PR closed (see last comment); continued as PR #1022

@wardbell Adding the DI cookbook. Let me know if this is what you had in mind.

Adding @PascalPrecht as well in case he has time to offer some feedback.

[Foxandxss]

We are promoting @Injectable() in every service (even if it has no dependency) as the best practice.

[thelgevold]

Thanks for pointing that out @Foxandxss - I will update if that's the direction we are going in.

That said, I am not sure I love that requirement. Mainly because it adds a dependency on Angular 2 where it isn't needed.

[thelgevold]

Based on team feedback I have added Injectable to all services for consistency

[wardbell]

Hi Tor -
I'm documenting here some of the more important changes that I'm making/recommending.

1. We recommend registering application-wide providers in the root component, not in bootstrapping. Anything that really belongs with the app itself goes in AppComponent. I'm moving your app-specific providers there. Illustrated usage by extending HeroBiosComponent with logger service injection.
2. We only register Angular overrides in the bootstrapper. A good example is router LocationStrategy or a dev-time http backend fake like a2-in-memory-web-api.
3. All component class names should end in Component. Fixed that.
4. We recommend that constructors be super simple and never call services that could do anything like hit a database. I revised the SortedHeroesComponent by moving the HeroesService call to ngOnInit.
5. I wanted to make a more dramatic argument against component inheritance so I made HeroesBase into a component. I also shifted the sorting logic to SortedHeroesComponent for ... ahem ... greater realism.
6. I think the inheritance issues are lower priority than anything else that you're showing. I shifted it to the bottom.

I'll update this comment as I make more changes.

[wardbell]

Hey Tor - I've made more changes but it's in a broken state. Please don't touch at the moment.

[wardbell]

AARGH. I renamed the branch to all lowercase (our standard), and it closed this PR.

Please see continuation PR #1022