SCHEMA-295. Ability to override reader schemas in Kiji tables

- introduces CellDecoderProvider.
- refactor TestHBaseKijiRowData to get rid of deprecated constructor.
- HBaseKijiRowData has almost only one constructor.
- CellSpec now has a CellDecoderFactory.
- Test Avro records (Node/Edge) have proper default values.
- CellSpec supports "no reader schema" (ie. use writer schema).

issue: https://jira.kiji.org/browse/SCHEMA-295
review: https://review.kiji.org/r/289