SCHEMA-425. KijiTableLayout.getDesc() should return deep copy

review: https://review.kiji.org/r/662
issue: https://jira.kiji.org/SCHEMA-425
SCHEMA-401. Update cell write path validation

Modifies AvroCellEncoder to perform schema validation on encode.

issue: https://jira.kiji.org/browse/SCHEMA-401
review: https://review.kiji.org/r/653