kiji-schema: SCHEMA-841. Implement KijiRowData ontop of KijiResult

issue: https://jira.kiji.org/browse/SCHEMA-841
review: https://review.kiji.org/r/2029
commit: https://github.com/kijiproject/kiji-schema/commit/d79354eb85158ef8032eee5538d581d2fb17c5ee