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.