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

issue: https://jira.kiji.org/browse/EXP-221
review: https://review.kiji.org/r/1363
commit: https://github.com/kijiproject/kiji-express-music/commit/436fdbac813e3492caa0244494ff48600845bb64