Form validator class: Validate HTML form field values

This object can be used validate HTML form field values.

It takes a form object and checks the class attribute of the form inputs to determine the type of validation that will be performed on each input.

The object implements several built-in types of validations but can register custom validation types.

Currently it supports built-in validation types for e-mail addresses, URLs and numbers.

Custom validation types can determine the type of event that triggers the validation, an error message to be associated to the validation and a function to perform the actual validation.