Clean up code and support pithos-hosted images

 * Drop "custom" image format type. The same functionality is provided
   by network-hosted images.
 * Images with id that starts with "http:", "https:", "ftp:", "ftps:"
   are considered to be network hosted and are downloaded via cURL.
 * Images with id that starts with "pithos:" are considered to be
   pithos-hosted.
 * Support for pithos and network hosted images is disabled by default.
   You may enable it when configuring the project  using
   --enable-pithos-backend and --enable-network-backend respectively.