Added the IvyForm added to the core classes

The form classes were supposed to be defined as a set of helper classes but after really using the framework to build this site and the CMS behind it, it became clear that it would be an essential part of any webbased system.

Although the IvyForm class is part of the core system it won't be loaded by default but can be loaded by Ivy's autoloading feature.

The main IvyForm class is set up using a set of handlers and decorator classes so it can be extended and defaults can be overriden.