kiji-modeling: EXP-221. Improve the KijiSlice and PagedKijiSlice APIs.

issue: https://jira.kiji.org/browse/EXP-221
review: https://review.kiji.org/r/1347
commit: https://github.com/kijiproject/kiji-modeling/commit/abd714c3e351bd6544e1b2245c96a2a6d725d424