kiji-express: EXP-127. Provide a trait users can extend to implement their own evaluate phase.

issue: https://jira.kiji.org/browse/EXP-127
review: https://review.kiji.org/r/1204
commit: https://github.com/kijiproject/kiji-express/commit/0399852c578418785444ecc91184c94d34dc656e