Value Bindings

For discussion:

I added some value bindings for my personal use. If there is interest in adding these I can add tests, documentation, etc.

They are to bind the "Selected value" from the `autocomplete` to the `viewModel` without writing a select event handler for the purpose each time.

``` html
<input data-bind="autocomplete: { source: PossibleColors, selectedValue: SelectedColor}" />
```

``` javascript
var viewModel = {
    PossibleValues: [ 'Red', 'Green', 'Blue' ],
    SelectedColor: ko.observable();
}
```

There are 3 new 'bindings'.

* `selectedItem`: Gets the ui.item
* `selectedValue`: Gets the ui.item.value
* `selectedLabel`: Gets the ui.item.label