For version 0.3:

downloader: proceed correctly when child processes die
resolver, cupt: querying about decisions
downloader: debdelta integration (or re-implementation)
worker: check disk available size before downloading
downloader: https protocol

For version 0.4:

source version: implement
binary version: implement translated description
cupt: implement 'why' command
cupt: implement 'why-not' command
shell: implement

