* BREAKING CHANGES for session passing

To make osin compatible with Google App Engine, the http request
needed to be pass in to Storage. Therefore these breaking change.

 * Change Storage.Clone() to add an interface{} parameter.
   This breaks previous implementation of Storage interface

 * Change NewResponse() and Server.NewResponse() to add an
   interface{}. This breaks previous code uses these interfaces.

 * Change NewResponse() and Server.NewResponse() to pass the
   interface{} to Storage.Clone()

The interface{} parameter is for passing session dependent data,
if needed.