add debug-level logging to FutureTransfer.listenFor

log all of the detailed progress events that are sent to the progress listener, as well as logging the means by which the `Promise` is completed (how the inherent race condition resolves).
minor: update the scratch code to test FutureTransfer
Merge pull request #21 from pellucidanalytics/topic/s3-progress-logging

add debug-level logging to FutureTransfer.listenFor
bump bintray-sbt plugin version

no need for custom resolver any more as the bintray sbt repo is mirrored into one of sbt’s default resolvers
bump version number to 0.6.1
Merge branch '0.6.x' as v0.6.1 release