(working but ugly) merge attempt of the latest marshaller introduction

Merge remote-tracking branch 'origin/release-2.3-dev' into w/init-http-routing

Conflicts:
	akka-http-core/src/main/scala/akka/http/model/HttpEntity.scala
	akka-http/src/main/scala/akka/http/marshalling/package.scala
	akka-http/src/main/scala/akka/http/unmarshalling/package.scala