add akka streams support

[raam86]

Getting spans that go through the http directives is nice but the application is still opaque since I am not using explicit actors.

My application is based on akka streams and slick and I can't find a way to weave these together.

[captainju]

I would like to use with akka stream too!

[raam86]

@captainju you can use this https://github.com/FindHotel/akka-stream-trace