Add the ability to return status codes as HTTP status codes
Summary
As per W3C standard, RESTful APIs should use HTTP status codes to return information that we currently store into the "status" key of every Response. When possible, and allowed by FatFree, we should give the client the ability to choose whether to return status codes in HTTP responses too, following the guidelines of this page: https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html