kiji-mapreduce: KIJIMR-255. Plug ColumnReaderSpec to allow KijiMR jobs to customize reader schemas

issue: https://jira.kiji.org/browse/KIJIMR-255
review: https://review.kiji.org/r/1475
commit: https://github.com/kijiproject/kiji-mapreduce/commit/67de36d71b2261b9df38e0bdbff55e9aa9824d45