For version 0.2:
doc: write tutorial
downloader, config: support per-host acquire settings
cupt, worker, resolver: reasoning for installing/removing packages (-D option)
cupt, worker: implement 'update' action
downloader: multi-url downloading
For version 0.3:
cupt: changelog action
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
