kiji-rest: REST-110. OpenTSDB collector for KijiREST metrics.

This change adds:
 - KijiRestClient.GetMetrics()
 - TSDB client.
 - TSDB collector for KijiREST metrics.

issue: https://jira.kiji.org/browse/REST-110
review: https://review.kiji.org/r/1680
commit: https://github.com/kijiproject/kiji-rest/commit/d831b1b593877d73e1f1ca778904e3cfbb680bc7