Add with_empty helper to help with Form::select

Prepends empty option to array. Used for Form::select.

The 3 practical use cases for this : 

```
Form::select('size', with_empty(array('L' => 'Large', 'S' => 'Small')));
Form::select('size', with_empty(array('L' => 'Large', 'S' => 'Small'), 'Select an option'));
Form::select('size', with_empty(array('L' => 'Large', 'S' => 'Small'), array('-1'=>'Select an option')));
```