Make HTML escaping optional for label contents. 

Form::label() applies HTML::entities to all label content. Sometimes this is not desirable, for example:

```
<label>Are you <strong>sure</strong> you want to proceed?</label>
```

This would currently be rendered as: 
```
<label>Are you &lt;strong&gt;sure&lt;/strong&gt; you want to proceed?</label>
```

Labels can also legitimately contain links and HTML entities. 

I propose adding an optional fourth argument to Form::label to allow the automatic escaping to be bypassed. 

```
public static function label($name, $value, $attributes = array(), $escape_html = true) 
```

The default would be to automatically escape content as at present, so compatibility with existing code would be preserved.