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);
  }
});
```

At the `"warn"` level, those constructors will continue to receive two
arguments, though the first one will be the union of the two. Identify
the constructors that expect two arguments and change them to expect
all attributes in a single, merged hash.

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.
Merge pull request #24 from zendesk/james/createChildView

add CREATE_CHILD_VIEW flag