Add Ember.Control

An Ember.Control is a view that is completely
isolated. Property access in its templates go
to the view object and actions are targeted at
the view object. There is no access to the
surrounding context or outer controller; all
contextual information is passed in.

The easiest way to create an Ember.Control is via
a template. If you name a template
`controls/my-foo`, you will be able to use
`{{my-foo}}` in other templates, which will make
an instance of the isolated control.

```html
{{app-profile person=currentUser}}
```

```html
<!-- app-profile template -->
<h1>{{person.title}}</h1>
<img {{bindAttr src=person.avatar}}>
<p class='signature'>{{person.signature}}</p>
```

You can also use `yield` inside a template to
include the **contents** of the custom tag:

```html
{{#my-profile person=currentUser}}
<p>Admin mode</p>
{{/my-profile}}
```

```html
<!-- app-profile template -->

<h1>{{person.title}}</h1>
{{yield}} <!-- block contents -->
```

If you want to customize the control, in order to
handle events or actions, you implement a subclass
of `Ember.Control` named after the name of the
control.

For example, you could implement the action
`hello` for the `app-profile` control:

```js
App.AppProfileControl = Ember.Control.extend({
  hello: function(name) {
    console.log("Hello", name)
  }
});
```

And then use it in the control's template:

```html
<!-- app-profile template -->

<h1>{{person.title}}</h1>
{{yield}} <!-- block contents -->

<button {{action 'hello' person.name}}>
  Say Hello to {{person.name}}
</button>
```

Controls must have a `-` in their name to avoid
conflicts with built-in controls that wrap HTML
elements. This is consistent with the same
requirement in web components.