One new feature and Bug fix

### Added the option to use a custom message when no results are found:
```ruby
autocomplete_field_tag 'address', '', address_autocomplete_path, :size => 75, 
:"data-autocomplete-label" => "Sorry, nothing found."
```

### Bug fix
Fixed a problem in the change function. It was trying to parse as JSON an undefined value when there is not ```data-update-elements``` attribute.

Fixed with:
```js
var update_elements = false;
if (jQuery(this).attr('data-update-elements')) {
  update_elements = jQuery.parseJSON(jQuery(this).attr("data-update-elements"));
}
```