Made it possible to have multiple preparers.

colander/__init__.py

@@ -1667,7 +1667,13 @@ class SchemaNode(object):
         appstruct = self.typ.deserialize(self, cstruct)
 
         if self.preparer is not None:
+            # if the preparer is a function, call a single preparer
+            if hasattr(self.preparer, '__call__'):
                 appstruct = self.preparer(appstruct)
+            # if the preparer is a list, call each separate preparer
+            elif type(self.preparer) is list:
+                for preparer in self.preparer:
+                    appstruct = preparer(appstruct)
 
         if appstruct is null:
             appstruct = self.missing