=str #15743 tests for prepend and append

* these have no AST nodes, they work with AST List directly
=str Sugar to make flows between vertices optional
Merge pull request #15846 from 2m/wip-{ap,pre}pend-tests

=str #15743 tests for prepend and append
=pro #15849 fix scala-graph dependency for building for Scala 2.11
=pro use idiomatic sbt for including post-scala-2.10 dependencies

This re-enables using `++ 2.11.2` in sbt for quick changing of Scala versions.
=pro upgrade macro paradise dependency to 2.0.1 (to allow building for Scala 2.11.2)
=str #15844 Use key labeled edges to support multiple edges between same vertices
Merge pull request #15855 from akka/wip-15844-key-label-edge-patriknw

=str #15844 Use key labeled edges to support multiple edges between same vertices
Merge pull request #15854 from akka/wip-flow-optional-edges-patriknw

=str Sugar to make flows between vertices optional
Merge pull request #15850 from spray/w/15849-fix-dependencies

Small build fixes to fix building for Scala 2.11.2
=str Make it possible to use graph cycles

* power user mode

* Dropping buffer in feedback loop