kiji-rest: REST-99. Forcibly terminate via KILL when TERM signal doesn't work

 - "kiji-rest-admin stop" now forcibly stops the KijiREST server on timeout.

issue: https://jira.kiji.org/browse/REST-99
review: https://review.kiji.org/r/1615
commit: https://github.com/kijiproject/kiji-rest/commit/18441691a0b6d8fa4f95b59b9ebcf6ca53cae47f