(CPR-8) Rake task for signing gems

This commit adds a sign_gem task into sign.rake that detached signs the
gem. It also ships the gems to the defined by builder_data gem_path and
gem_host.
@clantant The odd/even scheme is unique to mcollective these days and it's going to go away soonish. Can you toss a comment to that effect in here in the hope of reminding us to pull this logic out when the odd/even scheme dies?

Please & thank you.