Use .prop() instead of .attr() with jQuery

jQuery 1.6 introduced the .prop() method which replaces .attr() for
numerous cases. Using .prop() solves a number of issues experienced by
browsers when rendering with application/xhtml+xml MIME type.

Full information on why this change is necessary and the details of
.prop() and .attr() can be found at:
http://blog.jquery.com/2011/05/12/jquery-1-6-1-released/