Index.php fix for for the Footer, and APP_KEY suggestion

Fixed a short tag issue when trying to include footer.php (it now works for those of us who don't use short tags).

Also put in a suggested idea to make the APP_KEY a defined variable that you include in the index.php, that is then echoed in header.php.  Not the most elegant way to do this, but allows me to update to my APP_KEY faster.