Proposed changes for GAE compatibility

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 require a *http.Request
   This breaks previous implementation of Storage interface

 * Change NewResponse() and Server.NewResponse() to require a
   *http.Request. This breaks previous code uses these interfaces.

 * Change NewResponse() and Server.NewResponse() to pass the
   *http.Request to Storage.Clone()

This pull request fixes #28.