Base contact form class from which all contact form classes should inherit.
If you don't need any custom functionality, you can simply use
this form to provide basic contact functionality; it will collect
name, email address and message.

The ``contact_form`` view included in this application knows how
to work with this form and can handle many types of subclasses as
well (see below for a discussion of the important points), so in
many cases it will be all that you need.