kiji-schema: SCHEMA-588. ColumnReaderSpec: a specification of read properties on a column.

issue: https://jira.kiji.org/browse/SCHEMA-588
review: https://review.kiji.org/r/1252
commit: https://github.com/kijiproject/kiji-schema/commit/4c68540257568586b0e7cdbfa4962226100055c0