(CPR-8) Rake task for signing gems

This commit adds a sign_gem task into sign.rake that detached signs the
gem.
The line 151 problem has been fixed. The default right now is because we want to put our gems and tarballs, and their signatures,  in the same place right now. We also want the option to change to a different gem host and path if we decide to store the gems and their signatures in a different place.