Encourage decorator-style Ember.computed/Ember.observer

[HeroicEric]

• Use Ember.computed over property()
• Use Ember.observer over .observes()

fixes #455

[HeroicEric]

Wasn't sure whether or not to just completely remove this and the other one like it for .observes()

[michaelrkn]

@stefanpenner @mixonic @tomdale do any of you have any thoughts about this?

[michaelrkn]

Ooh, I have an idea. We could take this out of here and move https://github.com/emberjs/guides/blob/master/source/configuring-ember/disabling-prototype-extensions.md into this Object Model section. We'll just need to change the wording a bit to make it flow right. Does that seem okay?

[tomdale]

I am very much not in favor of integrating caveats about prototype extensions into the main flow.

I'd like to see us pick a happy path and talk only about that happy path in the primary documentation. Anything that deviates from the happy path can be moved to an auxiliary guide. If we make non-proto-extension syntax the default, let's update to that everywhere and create a legacy appendix describing how that syntax works for folks who need to work on code bases that use prototype extension.

[locks]

I second removing proto-extensions references from the guides, especially since we have versioning. If people think they were removed and can't use them, well we're halfway to our goal ;)

[michaelrkn]

Awesome!!! Let's get some feedback on your question and then get this merged in!

[HeroicEric]

Updated and removed the inline references to the old syntax in the guides for computed properties and observers.