- added possibility to set the model to use custom validation class

Its possible to set up custom validation class like so:

require_once 'someplace/myvalidatorclass.php'; // optional if autoload got namespace already
$cfg = \ActiveRecord\Config::instance();
$cfg->set_validator('\Appendix\Libraries\Validation');

custom validator class have to implement methods validate and get_record.

We use our own validator, that can validate also custom data, files etc., Don't want to have 2 different ways to validate stuff.