Fix for url loading of images in ofImage::loadImage

Re discussion in #3304, ofImage::loadImage did not work on https.
Switched from using a substring check for "http" in the filename to
using Poco::URI to detect http or https in the scheme.
Updaing changelog
Merge pull request #3308 from mikewesthad/bugfix-loadImage-HTTPS

Fix for url loading of images in ofImage::loadImage