Align README to akka-stream 0.7 upgrade
Run managed flows internally

Includes upgrade to akka-stream 0.8

In case of managed flows the user no longer has to run the flow.
To enable getting a sink's result another optional function taking
the MaterializedFlow can be supplied.

Additionally an implicit type conversion for FlowWithSource was
added for Flow -> Process.
Add more tests to stream.subscribe
Propagate errors from Process to Flow
Factor out common base class of adaptor-actors

Make AdapterPublisher and AdapterSubscriber extend common base-class
AdapterBase.

Includes some minor cosmetic changes.
Add tests that actors are stopped

Includes support for providing an actor-name for subscribe