Fixes EGI-FCTF/rOCCI-server#20, added CORS support

Added support for Cross-Origin Resource Sharing (CORS):
* Support for Access-Control-* and Origin headers
* Support for routing HTTP OPTIONS to a dummy controller
* Basic spec coverage
* Basic configuration options (enabling, setting origins)
* Disabled by default (possible security risk)

See `etc/*.yml` for details.
Merge pull request #25 from arax/cors

Fixes EGI-FCTF/rOCCI-server#20, added CORS support