kiji-mapreduce: KIJIMR-256. Kiji Pivot tool should have --start-row and --limit row options.

 - KijiPivot subclasses KijiJobTool instead of JobTool.

issue: https://jira.kiji.org/browse/KIJIMR-256
review: https://review.kiji.org/r/1492
commit: https://github.com/kijiproject/kiji-mapreduce/commit/78d9b0ee9359c5032f14a6197a4615a94e0d6075