feat(datepicker): accept disabled-dates

```html
<input bs-datepicker disabled-dates=“unavailableDates”>
```

Expects a sorted array of objects with `start`, `end` date properties.
```js
$scope.unavailableDates = [{start: new Date(2014, 3, 3), end: new Date(2014, 3, 5)}];
```

The array is expected to be immutable (it is not deeply watched).
The array is expected to be sorted.