kiji-express: EXP-152. Refactor Implicits object into a trait.

issue: https://jira.kiji.org/browse/EXP-152
review: https://review.kiji.org/r/1607
commit: https://github.com/kijiproject/kiji-express/commit/e01274e2023eded64894e50f9daf69713384043d