Refactored and cleaned up HTML, CSS and JS

Cleaned up index.html. Changes include:

* The page now validates as HTML5.
* HTML attribute values that don't need to be quoted were unquoted.
* Optional HTML opening and closing tags were omitted.
* Added new HTML5 tags where appropriate.
* CSS was modified to make the page slightly more responsive.
* All JavaScript was moved to the bottom of the page for faster loading.
* JavaScript functions were re-written using function expressions.
* JavaScript function is_int was refactored to be simpler.

Overall, it should be easier to maintain now.