add new API request.get_cleaned_form
This add a new API and enforce it (but with backward compatibility for now) to get access to form data from a HTTP request.
The api is:
request.get_cleaned_form(form_validator) and the
form_validator must provide a
.validate() method which will receive the form arguments as a dictionary.
This allow to have a generic API and the CW users will be able to use the framework of their choice (yams, colander, mashmallow, WTForms...)