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.
Make package filenames ruby 1.8/1.9 compatible
Clean up delete command.

Make command error out if nothing was deleted.
Make command delete all versions of PACKAGE by default.
Add path prefix option for S3 storage.
Merge remote-tracking branch 'delete/master'