Update system/libraries/Form_validation.php

CodeIgniter documentation states that rules in configuration can be specified by naming according to class/function.

This update maintains the current functionality using the uri->ruir_string() and adds new functionality consistent with the documentation by using the router->class and router->method. I do not check for the $controller_method to be empty as the router->class and router->method should always return a value.

[Associating a Controller Function with a Rule Group](http://codeigniter.com/user_guide/libraries/form_validation.html#savingtoconfig)
"...is to name it according to the controller class/function you intend to use it with..."