[BUGFIX release] Adds preserveContext option for #each helper

Since [3100](https://github.com/emberjs/ember.js/pull/3100) is kinda stale, I decided to take another stab at it.

It adds `preserveContext` property to `CollectionView` to fix problems `each` helper and makes `itemViewClass` property work with `ArrayController`.

Fixes #3086.

/cc @stefanpenner @wagenet @kselden