Datepicker: Allows disabled input to trigger datepicker

The Datepicker constructor should check the "disabled" property of its target input control and set its own disabled state accordingly.

When a webpage is rendered with the following markup:

<input type="text" name="date" id="mydate" disabled="disabled">

The Datepicker should also be rendered disabled when running the following code.

$("#mydate").datepicker({
	showOn: 'button',
	buttonImage: 'images/calendar.gif',
	buttonImageOnly: true
});