Support for easy JSON generation (via for example) Spray JSON

I am using ScalaBuff in a [Spray](http://spray.io) project where I output instances not just as Protobuf but also in JSON format. For this, I use [Spray JSON](https://github.com/spray/spray-json), which directly supports case classes – as long as they do not contain additional fields (i.e., fields that are declared in the case class body and not just in the constructor).

Unfortunately, `Parser` [has](https://github.com/SandroGrzicic/ScalaBuff/blob/9322a213ebc383d3deed6d291b3b956420983264/scalabuff-runtime/src/main/net/sandrogrzicic/scalabuff/Parser.scala) such a field: `final val EMPTY_REGISTRY`. This pull request proposes to make the field static by moving it to `object Parser`.