Add support for a system default credentials file.

The path is:
  - windows: %ProgramData%\Google\Auth\application_default_credentials.json
  - other: /etc/google/auth/application_default_credentials.json

googleauth.gemspec

@@ -35,6 +35,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'bundler', '~> 1.9'
   s.add_development_dependency 'simplecov', '~> 0.9'
   s.add_development_dependency 'coveralls', '~> 0.7'
+  s.add_development_dependency 'fakefs', '~> 0.6'
   s.add_development_dependency 'rake', '~> 10.0'
   s.add_development_dependency 'rubocop', '~> 0.30'
   s.add_development_dependency 'rspec', '~> 3.0'