Let the possibility to delete specific iteration of a package

e.g.:

deb-s3 delete --versions packageA-42
Fix `verify` and `delete`.
Introduce in 3e347127e99904e8fb279805798744fa36586275
Merge pull request #56 from guilhem/fix_Release.retrieve

Fix `verify` and `delete`.
Merge pull request #48 from remh/patch-1

Let the possibility to delete specific iteration of a package
Cleanup delete / add:
- Add distribution codename to url_filename Fix #55
- remove file from pool when delete
- remove package when NOT preserve-version