Bugs and fixes, add provision service accounts

• Add gar_scope_config
• Add Docker build available at gcr.io/gcer-public/googleauthr
• More API error feedback if using googleAuthR.verbose < 3
• Auto-auth should succeed if one of the scopes needed is present
• Set default scopes to "https://www.googleapis.com/auth/cloud-platform" in gar_gce_auth
• Corrected and clarified googleSignIn documentation (#171, @jonthegeek).
• Fix gar_debug_parsing() message
• Remove deprecated gar_auth_jsUI since it causes an issue when used
• Add functions gar_service_* to provision service accounts via R (#178)
• Remove OAuth2 login parameter approval_prompt as its replaced by prompt (#177 - thanks @OuNao/@rasmes/@hugovk)