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.