kiji-rest: REST-62. POST can increment counters.

Setting a counter cell value in a POST to {"incr" : 123} increments the
counter by 123.

issue: https://jira.kiji.org/browse/REST-62
review: https://review.kiji.org/r/1281
commit: https://github.com/kijiproject/kiji-rest/commit/8b1b2af677d7e408edd22aa7f8f32d571c040285