Switching from $.fn.attr to $.fn.prop for checked attributes

(This is copied from the commit)

"Toggle all" buttons do not work as expected on the [customize.html page](http://twitter.github.com/bootstrap/customize.html). Clicking them only un-checks the boxes; subsequent clicks do not re-check them.

Using $.fn.attr only retrieves the actual content of the "checked" attribute, so it never registers as false if there is content in the string. JQuery advises now using $.fn.prop for the checked attribute, which returns the actual value.

See: http://api.jquery.com/prop/