Document consuming multipart entities

Not only for the server-side (i.e. for consuming file-uploads) but also for the client-side where servers may return multipart/byteranges or other multipart datastructures.

Information it should contain:

• how to use Unmarshal(response / entity).to[Multipart.General]
• other subclasses of Multipart
• explain stream-of-streams nature of Multipart.parts and Multipart.BodyPart.entity.dataBytes
• note about head-of-line blocking: the data of a part needs to be read fully before a new part will be dispatched. Just filtering parts can deadlock a stream.