Adding delete action

This action removes the specified package at the specified version from
the list of packages in the repository. It does not delete the .deb file
itself from S3. Perhaps a purge command or something in fix_manifests
could list the orphaned .deb files.
Clean up delete command.

Make command error out if nothing was deleted.
Make command delete all versions of PACKAGE by default.
Update README for delete command.
Merge pull request #17 from quixey/support_deletion

Adding delete action