Add Form.__contains__() method

for testing if a field is contained in the form

tailbone/forms/core.py

@@ -364,6 +364,9 @@ class Form(object):
         self.action_url = action_url
         self.cancel_url = cancel_url
 
+    def __contains__(self, item):
+        return item in self.fields
+
     def set_fields(self, fields):
         self.fields = FieldList(fields)