Add support for protobuf

[pajapro]

A recent talk (Type-safe Web APIs with Protocol Buffers in Swift by Yusuke Kita) has inspired me to add a support for application/protobuf content-type for network responses. Doing so, would send the data binary encoded, which increases the performance (+). Also, the same models (.proto) could be used (generated) for client-side code (+).

On the other hand, Swift's new Codable protocol can ensure type-safety as well (one of the selling points of protobufs) -.

See Protocol buffers and Swift Protobuf