Correct URL encoding issue in afg_get_photo_url

The existing code caused thumbnails not to be displayed. The existing code nonsensically attempted to URL encode the "http://" portion of the URL. This change wraps the individual parameters with the URLencode() function while allowing the known protocol and URI control characters to pass through unescaped.
Merge pull request #12 from peteru/patch-1

Correct URL encoding issue in afg_get_photo_url