(CPR-8) Rake task for signing gems

This commit adds a sign_gem task into sign.rake that detached signs the
gem.
Why are we setting `gem_path` and `gem_host` to `tarball_path` and `tar_host`, respectively? In most cases, I think that will go to downloads.puppetlabs.com/opt/downloads/${project}. Also, there's a reference on line 151 of tasks/00_utils.rake that references `Pkg::Config.gemhost`. I'm not sure if that's on purpose, or if that's defined anywhere, but it seems weird to have both `gem_host` and `gemhost`