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.