Support the signing of multiple gems

[rochlefebvre]

Many gem commands accept multiple gems from the command line, like gem fetch and gem check. The Gem::Command super class exposes a method called get_all_gem_names to list them. Our implementation for gem sign uses get_one_gem_name, which aborts unless there is exactly one gem name argument.

Maintainers that publish a family of gems together (like Rails and rspec) would probably appreciate something like:

gem sign rspec-core.gem rspec-rails.gem rspec-mocks.gem, or even gem sign *.gem. Our GemSigner class could accepts 1+ gems as input. It would challenge the caller once for authentication, acquire one certificate around a key pair, then sign all gems with the key.