Model Validation: Validate form values according to many rules

This class can validate form values according to many rules.

It can take an array of rules that associate the name of form input values and the rules they must satisfy to be valid.

The class traverses the array of submitted values and checks if they satisfy the specified rules. All the validation errors can be returned in a single array.

Currently it supports validating the submitted values as alphanumeric text, as numbers, and as e-mail addresses.