ISPN-4867 Single event marshaller support more clearly defined in API

* Based on Dan's feedback, Hot Rod server API made it look as if you could pass a per-factory marshaller but that's not really the case. A single event marshaller is currently supported.
Or maybe [1]:
```
private def selectMarshaller = marshaller.getOrElse(defaultMarshaller)
```

[1] http://blog.originate.com/blog/2014/06/15/idiomatic-scala-your-options-do-not-match/