Support grouping option for Ember.Select using optgroup

I supported using `Ember.Select` with `optgroup`.

## Usage

To use `optgroup`, the `optionGroupPath` is required.

* Source

``` javascript
var content = Ember.A([
  { id: 1, firstName: 'Yehuda', organization: 'Tilde' },
  { id: 2, firstName: 'Tom', organization: 'Tilde' },
  { id: 3, firstName: 'Keith', organization: 'Envato' }
]);

var select = Ember.Select.create({
  content: content,
  optionGroupPath: 'organization',
  optionLabelPath: 'content.firstName',
  optionValuePath: 'content.id'
});

select.appendTo('body');
```

* Result

![ 2013-04-14 2 53 44](https://f.cloud.github.com/assets/290782/376916/2221c1b4-a463-11e2-9888-4ba501eb6004.png)


*caution: `content` should be sorted by `optionGroupPath`*.