SCHEMA-530. Implement developer validation mode

- CellSpec has an optional column URI.
- AvroCellEncoder require the column URI from the CellSpec.
- AvroCellEncoder registers writer schemas on demand (DEVELOPER mode).
- Adds unit-test for developer validation.

issue: https://jira.kiji.org/browse/SCHEMA-530
review: https://review.kiji.org/r/989