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

issue: https://jira.kiji.org/browse/EXP-221

contributed-by: akahn (adam kahn, akahn@wibidata.com)
review: https://review.kiji.org/r/1341
commit: https://github.com/kijiproject/kiji-express/commit/3f424d980f80838588899178bf2fdd27d2508f02