Various best practice fixes to samples:
- Switched to client secrets where appropriate
- Switched to installed application flow (google/api_client/auth/installed_app) where appropriate
- Added caching of API definitions where appropriate
- Added application name and version to client initialization
- Removed outdated OAuth Util (superceded by google/api_client/auth/installed_app)
Merge pull request #3 from sgomes/discovery_caching

Various best practice fixes to samples