Misc fixes and enhancements, including:
- Support for additional claims in JWT tokens (#171)
- Adds makeHttpClient from credentials (#162)
- Remove classmap autoloading from composer.json (#168)
- Fixes wrong variable name in Cache (#161)
- Remove extra call to isValidKey in Cache (#167)
- Ensures onAppEngine returns true on Dev AppServer (#170)
- Fixes README base_uri for guzzle6 (#143)