Add content caching capability to 3scale APICast gateway

Nginx supports supports the inclusion and setting of cache control headers in its ngx_http_headers_module: http://nginx.org/en/docs/http/ngx_http_headers_module.html

This RFE is to allow the "Expires" and "Cache-Control" headers to be set, per 3scale API, on the 3scale Manager's Integration screen and using the 3scale APIs.