Add CORS in POST response

Changes:

- Added Access-Control-Allow-Origin in POST response headers
- Entity is returned as POST response body as per HTTP1.1 spec suggestion (http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.2.1)
- Inside CORSregisterInstallation, the Access-Control-Allow-Headers header value is filled automatically from the request, instead of hardcoding the headers