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

issue: https://jira.kiji.org/browse/EXP-353
review: https://review.kiji.org/r/1536
commit: https://github.com/kijiproject/kiji-express/commit/b9fdfeecfce10d8918fe9923be359113c327e21e