Working on documentation and packaging
Added a User-Agent header to backend HTTPS request message
Added a pre-processing decorator at ``api.response.message_extractor``

This is to aid custom ``response_callback`` implementation for use in
application bindings like ``wsgi.InterceptorMiddleware`` and
``cgi.InterceptorController``. It simplifies the need to pre-process
response body data and ensures consistency through hiding details.
Merge remote-tracking branch 'liuyu/develop' into develop