Respect :id option when generating javascript_tag

Makes this work properly:

````
= f.datepicker :date, id: "custom_id"
````

Before this you would get:

````
jQuery(document).ready(function(){jQuery('#model_name_date').datepicker({})});
````

Instead of:
````
jQuery(document).ready(function(){jQuery('#custom_id').datepicker({})});
````

There's also a call to stringify_keys in there for supporting symbols, so that both of these work:
````
= f.datepicker :date, id: "custom_id"
= f.datepicker :date, "id" => "custom_id"
````