Add an option to disable SSL certificate validation

The default service provider //ckeditor.iframe.ly/api/oembed?url={url}&callback={callback} uses a invalid SSL certificate. (which is only valid for iframe.ly and www.iframe.ly, not ckeditor.iframe.ly).

This results in cURL failing to validate the certificate, and therefore failing to resolve the embeded media.

Because such situation (i.e. service provider using invalid / expired certificate) is not under our control, it is better to have an option to disable certificate validation, or to force HTTP on all requests.