SCHEMA-372. FromJson should accept integer nodes as valid long (and double) nodes

issue: https://jira.kiji.org/browse/SCHEMA-372
review: https://review.kiji.org/r/508
SCHEMA-350. Export metadata backup sets system table.

Also added TestMetadataTool.

issue: https://jira.kiji.org/browse/SCHEMA-350
review: https://review.kiji.org/r/369/
reviewer: taton, ajprax