added app/Config/core.php.default and .gitignored app/Config/core.php
Just like the approach for app/Config/database.php.default.
Because CakePHP requires Security.salt and Security.cipherSeed in the core.php file to be altered from its default values before launching.
Updated error message in lib/Cake/Core/Configure.php for the scenario where CakePHP can't find core.php.