Added CORS support to 'ping', 'get images' and 'get tags' methods in registry REST api

I’m trying to build a [web interface](http://l3n41c.github.io/private_docker_registry_browser) on top of docker registries in order to provide a convenient way to browse the content of private registries.

In order for the browser to be able to use the REST API, it needs to implement [CORS](https://en.wikipedia.org/wiki/Cross-origin_resource_sharing).

This problematic has already been described in #333.
The CORS support has already been implemented for the “search” API in #338.

This pull request adds CORS support for the “ping”, “list tags” and “image info” APIs.