Add icon to prepend input field

This request adds the possibility to add an icon to input fields as shown in http://twitter.github.com/bootstrap/base-css.html#icons under the Form fields section.
This is done by extending the prepend input behaviour.

To add an icon to your text/number input field, simply add an array to prepend input with the icon name.

Example:
```php
$builder
->add('start_date', null, [
                'widget' => 'single_text',
                'format' => 'yyyy-MM-dd',
                'attr' => [
                    'class' => 'datepicker span2',
                    'prepend_input' => [
                        'icon' => 'icon-calendar'
                    ]
                ]
            ]
);
```