Fix incorrect arguments of calling add_cookie_info

TestClient.add_cookie_info() actually takes a request with
keyword arguments, but TestClient.perform_request() was passing
two positional arguments.
Add Website.wsgi_app hook for WSGI middleware
Use StringIO instead of io module for 2.5 compat
Merge pull request #183 from dahlia/wsgihook

Add Website.wsgi_app hook to utilize WSGI middlewares