Provide a way in response to fetch the response completion timestamp.

[prat0318]

It is sometimes necessary to find the time taken by the client call, but finding it for an async call is hard at application level because the response might have come much before the result() inspection is done.

I propose adding another param to Response ctor here:

self.finish_timestamp = time.time()

Which can then be read by application logic, eg. :

>>> client = SwaggerClient.from_url("http://localhost:12343/swagger.json", FidoClient())
>>> a = client.hours.getHours(biz_id=1)
>>> time.time()
1443127426.439981
>>> a.future.result().finish_timestamp
1443127423.185333

Which shows that finish_timestamp was recorded before result() (the blocking call) was called.