Use relative protocols for SSL

I've changed the path for renderTag to use only "//" instead of "https://" when ssl option is true. This allows the resources to be loaded based on the URL of the browser, so when browsing http sections of a site, the CDN will be referenced using http (thus incurring lower costs) and https request will be issued only for https urls.

Also I've included a section on the readme on how to configure SSL to work with cloudfront. This took me some time to figure out, so I guess it wouldn't hurt having that around.