kiji-modeling: EXP-298. Rename ColumnRequest[Input|Output] to Column[Input|Output]Spec

issue: https://jira.kiji.org/browse/EXP-298
review: https://review.kiji.org/r/1370
commit: https://github.com/kijiproject/kiji-modeling/commit/353694c111ccc8ac5a7122e5f70c697348b34fb9