Easily add the ability to use model methods as Form Validation rules.

-Adds one public method to set a model name or array of models that will be checked for rules.
-Slightly modifies the _execute() method to check for rules in models before continuing the hunt.

Originally I added a simple public method to Loader() which would return the protected $_ci_models array to see if it was already loaded but thought just trying to load the model was less intrusive as it will just return the model if it is or load and it if it isn't.

I will gladly amend the related docs if this change is accepted.