Bug fix: support non-pickleable schemas

Non-pickleable schemas are not uncommon.  (Compiled regular
expressions --- used, e.g., in the colander.Regex validator --- are
non-pickleable.)

Copy.deepcopy does not work with non-pickleable objects.
Bug fix: make sure to rebind the schema to each request
Merge pull request #197 from dairiki/bug.schema-binding

Bug: Schema is only being bound to the first request