Sets the HTTP Access-Control headers to allow for Cross-Domain Ajax requests. Only supported in the most modernest browsers.

See https://developer.mozilla.org/en/HTTP_access_control for details on Access Control

Sets an X-Runtime header in the response to get an idea of how long a request takes, excepting transfer and connection overhead.

Uses HiTimes for the timer, but could easily enough use Ruby's Time class to do the same.