Adding settings for CDN compatibility.

Please do not merge this while I continue to work on it, just wanted to make this Pull Request so you can see what I'm doing.
Not sure why this shows up as a new addition. We're using $mybb->cache ina  few places:

- usercp.php:3044: `$usergroups = $mybb->cache->read('usergroups');`
- inc/functions_user.php:590: `$usertitles = $mybb->cache->read('usertitles');`
- inc/init.php:208: `$mybb->cache = &$cache;`
- install/upgrade.php:133: `$mybb->cache = &$cache;`