Added a request and response file to common

Added two files (request.py and response.py) to marconi/common that
will stage communication between the transport layer and new API
layer. Also added a test to request in tests/unit/common.

Partially-Implements: cross-transport-api-spec

Change-Id: I2d67a15fa8c6aef23ebcde6a71dda3fcb723873a
Merge "Added a request and response file to common"