kijiproject.github.com: EXP-355. Add support for Key-Value stores in KijiExpress Flows.

issue: https://jira.kiji.org/browse/EXP-355
review: https://review.kiji.org/r/1542
commit: https://github.com/kijiproject/kijiproject.github.com/commit/5a5a971c4c7c0931106bb6b70b7652b24470a212