Add itemView instead of getView in CollectionView.

[paulmillr]

This would make stuff more Backbony.

@karellm review please

# Was.
class UsersView extends CollectionView
  getView: (model) ->
    new UserView({model})

# Now.
class UsersView extends CollectionView
  itemView: UserView

# Or even.
users = new Collection null, {model: User}
usersView = new CollectionView {collection: users, itemView: UserView}

[molily]

Good idea, keeps simple use cases easy. The user can still override getView if a more complicated algorithm is needed.

[molily]

Probably change this error message to the itemView property must be defined (or the getView must be overridden) or something similar.

[paulmillr]

done @ 7852b76