Respect the autocomplete attribute for forms

deform allows creating a form as follows:
    >>> class MySchema(colander.Schema):
    ...     foo = colander.SchemaNode(colander.String())
    ...
    >>>
    >>> form = deform.Form(MySchema(), autocomplete=False)

The generated form should have a 'autocomplete="off"' attribute set as a
result, but it was ignored by the form.pt template.

This change is inspired by the code of the form.pt template in deform
itself.