Add extension loaded check

[thekabal]

PHP allows us to check if the needed extensions are loaded:

http://php.net/manual/en/function.extension-loaded.php

Checking for pdo, mbstring, and a DB extension would be ideal, with well-formatted errors to let the admin know.

[thekabal]

Also, check the PDO db driver is available: http://php.net/manual/en/pdo.getavailabledrivers.php