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.)
You mean the underscores instead of camelCasing?
We had a quick vote a while back on how we want to translate things like `application/atom+xml` from Scala (which uses the "original" in backtick notation) to Java and settled on this (AFAIR it was @rkuhn's) proposal to use underscores.
It's not too late though to change this now if you have a strong preference.
If you have a strong