Module support with hdfs mode in scald.rb

With this PR, it is now possible to run the recently committed tutorial material (which require sclading-json and/or scalding-avro) in :hdfs mode without first building fat jars.  (Happy side-effect:) It also allows our budding local production (thin) jobs to run in :hdfs mode as well.  The orthogonal issue (#787) is not addressed here.

Before working on this, I noticed and resolved the duplicate mention of options; and noticed reference to sbt 0.12 (which is no longer supported as a top-level build option).