Use an aggregator project

This doesn't change the way the samples projects are defined (hence you can copy
paste them out of the master project and they will still work I kept the unused
plugins.sbt files in the samples on purpose)

However it should make developers life easier as they won't have to remove the
ivy cache, republish a snapshot locally and rerun the sample project to test
changed. A developer can now use `project scalaDemo` or `project javaDemo` to
activate the wanted project then run. Any change in the sample project or in the
module-code project will trigger an application reload effectively making
modifications available immediately.