Implement the following classes, including tests:

KeyInfo
lib/googleauth/id_tokens/key_sources.rb -> lib/Google/Auth/IDTokens/KeySources.pm
test/id_tokens/key_sources_test.rb -> t/key_sources_test.t

Verifier
lib/googleauth/id_tokens/verifier.rb ->  lib/Google/Auth/IDTokens/Verifier.pm
test/id_tokens/verifier_test.rb -> t/verifier.t

WebUserAuthorizer
lib/googleauth/web_user_authorizer.rb -> lib/Google/Auth/WebUserAuthorizer.pm

UserAuthorizer
lib/googleauth/user_authorizer.rb -> lib/Google/Auth/UserAuthorizer.pm

DefaultCredentials
lib/googleauth/default_credentials.rb -> lib/Google/Auth/DefaultCredentials.pm

ServiceAccountCredentials
lib/googleauth/service_account.rb -> lib/Google/Auth/ServiceAccountCredentials.pm

ClientId
lib/googleauth/client_id.rb -> lib/Google/Auth/ClientId.pm

GCECredentials
lib/googleauth/compute_engine.rb -> lib/Google/Auth/ComputeEngine.pm

IAMCredentials
lib/googleauth/iam.rb -> lib/Google/Auth/IAMCredentials.pm
