Fix for url loading of images in ofImage::loadImage

Re discussion in #3304, ofImage::loadImage did not work on https.  At @bakercp's suggestion, switched from using a substring check for "http" in the filename to using Poco::URI to detect http or https in the scheme.

Tested on VS2012 using this [code](https://gist.github.com/mikewesthad/ef4748777ec84a5fdba5), which checked loading images using the following:
- https (case insenstive)
- http (case insenstive)
- images in data folder
- images specified by absolute path
- images specified by relative path

Hope this is all kosher