Modify Servme::ServiceStubbing#respond_with to take a status_code

Allows service stubs that return a specific response AND a specific
status.
Make Servme::Responder#respond work for JSON requests

Sinatra requests don't parse JSON attributes and convert them to params.
Since we needed to be able to test JSON api requests, I've added this
capability to #respond.
Merge pull request #5 from kgodard/master

Updates to Responder and ServiceStubbing