Java Http Model

Follow-up to typesafehub/akka-streams#66 based on Mathias' latest version. Alone because of the sheer amount of classes I'm pretty sure there are some rough edges left. So, please be rigorous. 

(It seems this PR has to live in the spray/akka repository as long as its base is also living there.)
I understand. This is needed if we want to use the same data model, which we want for performance reasons. I don't like it, but I don't see a way around it. Thanks.