Autoloader

This is my proposed solution to #780. Basically we remove the autoloader entirely from dompdf_config.inc.php, and let the developer make their own decision whether or not to use it.

Unfortunately I didn't realise you were so close to releasing 0.6 final, it would have been nice to make this change before the release. As you said in 0.7 you are planning to make everything PSR0 perhaps you can drop the autoloader entirely at that point (or just ship a default PSR0 autoloader).