Given the contents for an image, returns a data URI string
representing the data in that image.
Adapted from:
https://github.com/jbkirby/css-img-datauri-stream/blob/master/index.js
function(imageFile,mimeType) {
                // Convert it.
                var ret = 'data:';
                ret += mimeType;
                ret += ';base64,';
                ret += imageFile.toString('base64');
                return ret;
            }