Add sbt key to set path to docker binary

The path to a docker binary can be set using the sbt key 'docker-path'.
By default the value on environment variable 'DOCKER' will be used, if it does not exist 'docker' is assumed to be on the path.
Update to ScalaTest 2.1.0
Add support for Dockerfile instruction ONBUILD
Change task keys to sbt 0.13 naming style