Validation classes

I have added some validation classes to the documentation section.

The validation classes can be used on their own but they are intended to be used as part of the XML form builder, shortly I will update the example for the lib_form_builder_xml class to show correct usageĀ and also add the processing class code and example.

The processor class is the one that reads the rules from the XML file and actually validates the submitted data before it can be passed of to the model/controller.