kiji-express: EXP-355. Add KeyValueStore support to Express Flow.

review: https://review.kiji.org/r/1509
issue: https://jira.kiji.org/EXP-355

review: https://review.kiji.org/r/1509
contributed-by: clint (Clinton Kelly, clint@wibidata.com)
commit: https://github.com/kijiproject/kiji-express/commit/b7ef9512f79c569c2a208f3330b6420bf26712e3