Validation Engine: Validate text values according to many rules

This class implements many rules for validating text field values.

It can be used to validate field values submitted using Web forms.

It can return a validation summary report using customizable error messages style.

Currently it supports the following validation types:

- Numeric within a range
- Regular expression match
- Email address
- Date
- Comparison of two field values,
- Text with a minimum and maximum length
- Custom for user-defined validation

Each text field value may be required or not and have associated individual error messages.