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

issue: https://jira.kiji.org/browse/EXP-353
review: https://review.kiji.org/r/1537
commit: https://github.com/kijiproject/kiji-modeling/commit/4ca1347ed3622ea20edbae18f0db8b6b32b4d517