* StreamTcp and Http extensions now return Flows and Sources that can be materialized later
* Flow can now be completed with another flow to be turned into a runnable flow
* implicit param
* change tests to use implicit materializer
* add ActorFlow trait that provides an implicit materializer inside an actor
in the right way, i.e. encourage usage of that instead of
implicit def mat(implicit arf: ActorRef): FlowMaterializer
* make http compile, but those who know the api better will have to adjust
to take full advantage of the implicit materializer
