Improving extensibility of the main Restler class

    Moving the authentication and 'call API method' parts from the Restler::handle() method to external methods, so they can be easily extended if needed.

I felt the need of catching some exceptions that are thrown throughout my API classes in a higher place than repeating the same try..catch pairs on and on. Thought about extending the Restler class to achieve this, but the code was packed into one method. Thus, I modularized it a bit and extended the class like this:

```php
class Base extends \Luracast\Restler\Restler {

	protected function runApiMethod($o, $accessLevel) {
		try {
			return parent::runApiMethod($o, $accessLevel);
		}
		catch (\ActiveRecord\RecordNotFound $e) {
			$this->handleError(HTTP_NOT_FOUND);
		}
	}

}
```