If the input field has the disabled attribute the datepicker won't show

In both situations:
```html
<div class="input-group date" id="from" data-date-format="dd/mm/yyyy">
	<input class="form-control" size="16" type="text" name="start_date" id="start_date" required disabled>
	<div class="input-group-addon">
		<span class="glyphicon glyphicon-calendar"></span>
	</div>
</div>
```

```html
<input class="form-control" size="16" type="text" name="lol" id="lol" required disabled>
```

adding the disabled attribute on the input field make the datepicker not shows himself.

I've also added a new method to enable the field
```js
$('#id').datepicker('enable');
```

thnx for your attention