Support multipart messages

[gafiatulin]

With current version it is impossible to differentiate between simple and multipart messages.

One way to do that is to treat all messages as multipart: simple being multipart with only one part.
Source[ByteString] → Source[org.zeromq.ZMsg]

Or introduce own message type ZMQMessage:

sealed trait ZMQMessage
case class SimpleZMQMessage(bytes: ByteString) extends ZMQMessage
case class MultipartZMQMessage(parts: Seq[ByteString]) extends ZMQMessage

And use Source[ZMQMessage]