The Http::Response class now defaults to a 0 status code.

A 0 status code indicates that no response was received.  This is the
case for networking errors.
Removed a require statement that was not needed.
Removed the concept of signalling a response as complete.

Response callbacks are now just helpers for handling responses
with specific status codes.
Updated handlers to stop using #signal_complete on the Response.
Added a travis configuration file.