Refactor PredictionJob to use MultipleInputs for reduce side joins

The code in org.apache.mahout.cf.taste.hadoop.als.PredictionJob should be refactored to use org.apache.hadoop.mapreduce.lib.input.MultipleInputs for the reduce-side joins. This should spare us some M/R cycles and greatly simplify the code.

We'd need to add another prepareJob() method to AbstractJob in order to make this work.

This is a rather cosmetic feature request that can wait till after the 0.5 release.