SCHEMA-420. KijiTableLayout#validateCellSchema tolerates null schema values.

review: https://review.kiji.org/r/652
issue: https://jira.kiji.org/SCHEMA-420
reviewers: taton,robertchu
SCHEMA-407. Provide easy to use helper classes around pagers.

Adds iterators wrapping KijiRowData.getPager() to:
 - iterate through the versions in a column;
 - iterate through the qualifier names in a map-type family;
 - iterate through the versions in all columns in a map-type family.

issue: https://jira.kiji.org/browse/SCHEMA-407
review: https://review.kiji.org/r/615
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