JSONDecoder is ambiguous for type lookup

[pronebird]

JSONDecoder is now a part of Foundation on iOS 11 and there is no way to use JSONDecoder anymore because there are now two of them. I have a code that extends both JSONDecoder and JSONEncoder and both throw an error on Xcode 9 beta:

'JSONDecoder' is ambiguous for type lookup in this context

It seems like JSONCodable will have to prefix its classes.

[matthewcheok]

Ideally, this library would become obsolete and we'd migrate to the new built-in Codable protocol in Swift 4. The only possible issue would be if we wanted to support multiple versions of iOS - it's possible the new API is available retrospectively since the Swift runtime is bundled with the application but I have yet to test this.

[emilwojtaszek]

I've fixed this in #82