kiji-express-music: EXP-353. Rename utility classes to *Util.scala.

issue: https://jira.kiji.org/browse/EXP-353
review: https://review.kiji.org/r/1538
commit: https://github.com/kijiproject/kiji-express-music/commit/42f5484b4d1730309ede16914ad18d6cbac325d9