Add deleting package feature

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
```