feat(ngForOn): add ngForOn directive with async support

[0x-r4bbit]

@robwormald came up with this via @jhusain' s implementation of it.

It's basically like ngFor but it resolves observables automatically. Here's the implementation + example ~ http://plnkr.co/edit/GcCqn1?p=preview

I wonder if we could add this to the core?

[0x-r4bbit]

I think this can be closed as this can be implemented with NgFor and AsyncPipe

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