Alternative to ResponseHandler's Requirement of JSONInitializable

[ghost]

At this time, the ResponseHandler protocol process method requires that the Output generic type conform to the JSONInitializable protocol. This is not ideal, as we could want to create other response handlers than simply JSON (such as plist, csv, raw text, etc.).

As of Swift 3, protocols can have associated types, but these require concrete types and may not themselves be protocols. Ideally, associated protocols, whereby the Output type could be constrained by, would be preferable.

Alternatives should be explored and this protocol definition made more flexible and clear.