Support for 404 page and 500 page.

Hi Panique, I think in the project, we might need custom error pages. I created custom exceptions to handle 404 error page and 500 internal server error page (actually it is also possible to make more exceptions to handle other http errors). And I caught the exceptions in index page, and response with appropriate header. Developers can throw an Internal server exception whenever it is applicable (eg. PDOException or any other exception) and it will be caught in index page to redirect to the corresponding error page. 

And, I added DEFAULT_CONTROLLER and DEFAULT_ACTION attributes in config file, so that if user enter just URL (eg. http://localhost/php-mvc), it won't throw a page not found exception and call the default controller and action.

Any feedback is welcome. Cheers.