User-defined protocol

You can now use the protocol parameter to specify which protocol should be used for the image URL. Can be useful in case you call your script from a local tool over HTTP but server images over HTTPS, or if you call the API over HTTPS but don't have HTTP support on another domain name you want to use for displaying the images.

This completes the domain parameter from v1.3.0 f364218 and has the same behavior when unspecified: assume the protocol is the same than the one used to call the API itself.