Sequential Song Gatherer

This is the rest of the base code for the music recommendation tutorial. Needs some polish, but we can iterate on that in conjunction with the written portion.

Still to do:
-More comments needed.
-Package level jdoc, and/or checkstyle config needed.
-Test for RecommendationProducer and IdentityMapper + TopNextSongsReducer needed.

We should get this under continuous integration tomorrow. (Kimball said he will do this tomorrow)