Support RawRepresentable enums out of the box about stork

[NunoAlexandre]

Done for Enums that areString or Int representable.

Now such an Enum is only required to state to comply to FromJson:

// That's all you need for String/Int RawRepresentable Enums!
extension Subscription: FromJson {}

Whenever the trivial Enum.init(rawValue: [Int|String]) way of decoding is not the way, the developer has full control by implementing its own custom from from(value: JsonValue) -> MyEnum?:

// Or you get to say how it's done!
// In this case, the country in JSON is short-coded and
// thus needs to be translated to the right Country case.
extension Country: FromJson {
  static func from(value: JsonValue) -> Country? {
    return value.stringValue().flatMap { str in
      switch str {
      case "nl": return .netherlands
      case "pt": return .portugal
      default:   return nil
      }
    }
  }
}