support HTML5 form attribute

By searching for input elements also outside the form tag itself, which belong to this form according to their `form` attribute.

closes #8

Author: JohaJung

webtest/forms.py

@@ -432,7 +432,10 @@ def _parse_fields(self):
         fields = OrderedDict()
         field_order = []
         tags = ('input', 'select', 'textarea', 'button')
-        for pos, node in enumerate(self.html.find_all(tags)):
+        for pos, node in enumerate(set(  # convert to set to remove duplicates
+            self.html.find_all(tags)
+            + self.response.html.find_all(tags, attrs={'form': self.id})
+        )):
             attrs = dict(node.attrs)
             tag = node.name
             name = None