Play JSON support for JSON-RPC

Usage:

1. import com.lightform.mercury.json.playjson._
2. Any time a Reader[JsValue, A] or Writer[JsValue, A] is needed as an implicit parameter, just ensure you have a Reads[A] or Writes[A] in implicit scope.
   readsReader and writesWriter defined in PlayJsonDefinitions will do the conversions for you.