Make Akka Wamp not explicitly depend on Akka Stream

[pangiole]

For reasons explained in the Akka HTTP docs, it would be good to make also Akka Wamp not explicitly depend on Akka Stream anymore, but rather build it using the provided SBT scope.

Following is an excerpt of the above Akka HTTP docs, adapted to the akka-wamp case:

Users will always have to add a manual dependency to akka-stream. Users will make sure they have chosen and added a dependency to akka-stream when updating to the new version of akka-wamp (veterans may remember this policy from Spray).

[pangiole]

Implementing this enhancement may require to split the core module into three separate modules: core, router and client