Merge pull request #340 from tbetbetbe/grpc_ruby_add_signet_based_auth

src/ruby/Rakefile

src/ruby/bin/interop/interop_client.rb

src/ruby/bin/interop/test/cpp/interop/messages.rb

src/ruby/bin/math.rb

src/ruby/bin/math_services.rb

src/ruby/grpc.gemspec

src/ruby/lib/grpc.rb

src/ruby/lib/grpc/auth/compute_engine.rb

src/ruby/lib/grpc/auth/service_account.rb

src/ruby/lib/grpc/auth/signet.rb

src/ruby/spec/auth/apply_auth_examples.rb

src/ruby/spec/auth/compute_engine_spec.rb

src/ruby/spec/auth/service_account_spec.rb

src/ruby/spec/auth/signet_spec.rb

src/ruby/spec/channel_spec.rb

src/ruby/spec/generic/active_call_spec.rb

src/ruby/spec/spec_helper.rb