Add example for federated gateway

Changelog for v0.2.3:

• PR #10 Add example for federated gateway by @alexellis
• PR #9 Add reference for ServiceAccountTokenSource helper in README by @welteki

Commits
8d9ce49 Update example to be more generic by @alexellis
24d4cde Use the TokenSource interface for the client credentials exchange by @alexellis
9fcabb0 Add example for federated gateway by @alexellis
73f84ff Add reference for ServiceAccountTokenSource helper in README by @welteki

Changes: v0.2.2...v0.2.3

Generated by Derek