Added support for an optional Placeholder Tag

Just added in the ability to set a placeholder by utilising the disabled selected $placeholder attribute.

The usage is like so: 

```php
Timezone::selectForm($selected, $placeholder, $formAttributes, $optionAttributes);
```

and it outputs the following html when the form is rendered:

 ```html
"<option value='' disabled selected>$placeholder</option>"
```

Thanks a lot for making this package. This minor addition is the least I could do to say thanks!!