add CREATE_CHILD_VIEW flag

In Ember 0.9, `Ember.View#createChildView` calls

```javascript
view = view.create(coreAttrs, attrs);
```

In Ember 1.0, it merges those "core" attributes into `attrs` and calls
`create` with a single argument.

Thus, if your `itemViewClass` class *expects* multiple arguments, it
will cease to work in Ember 1.0.

Setting `ENV.CREATE_CHILD_VIEW` to `"warn"` will tell you where your app
has child view classes that look like

```javascript
MyChildView = Ember.View.extend({
  ...
}).reopenClass({
  create: function(coreAttrs, attrs) {
    this._super(coreAttrs, attrs);

    doSomethingWith(coreAttrs._parentView);
    doSomethingElseWith(coreAttrs.templateData);
  }
});
```

You will have to change those constructors to accept a single hash
argument. The single hash will be the union of what used to be
`coreAttrs` and `attrs`.

After you have eliminated all warnings, you can set
`ENV.CREATE_CHILD_VIEW` to `"1.0"`, which will cause
`Ember.View#createChildView` to throw an exception (via
`Ember.assert`) if your view constructor expects multiple
arguments.