Create implementation class for simple S3 release
add callbacks for after upload so we can add UI
Always attempt to load config, message if there isn't one
implement `release package` command (rm placeholder)

Adds `release` namespace with `package` subcommand. The intent is to
also add a omnitruck-compatible release command (currently in the
jenkins/ directory of omnibus-chef and -server repos) in this namespace.

Implemented package command releases a single package with associated
metadata file to a single S3 bucket.