kiji-rest: REST-41. Add support for CORS.

Global cross-origin resource sharing can be turned on with:
"cors": "true"
in the configuration.yml.

issue: https://jira.kiji.org/browse/REST-41
review: https://review.kiji.org/r/1601
commit: https://github.com/kijiproject/kiji-rest/commit/fa2ad2cd0f569afb1bbef21d385b9110f5df060d