added app/Config/core.php.default and .gitignored app/Config/core.php

**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.

Also updated error message with a ToDo guideline in `lib/Cake/Core/Configure.php` for the scenario where CakePHP can't find the `core.php` while launching for the first timea.