Add deleting package feature

[fangpenlin]

I made a stupid mistake, I add a tag test-build for testing building pipeline, and then a new .deb file was uploaded to our apt server. Since the version test-build always greater than version number like 1.0.1, so I always get the test-build as the latest version. I decided to remove that .deb file from server, I modify this tool to make it can delete a package.

To delete a package with arch amd64, package name is billy and version is test-build-1, here you can run

depot -s s3://apt.example.com -c precise -k 55EA7812 --component=unstable --no-public -a amd64 --delete billy@test-build-1