Update JWT auth client with impersonation support

This patch upgrades the **gapitoken** library to newest version which adds support for the "sub" attribute in JWT authorisation header which is necessary to perform impersonation.

**Notes:**

 - I have relaxed the package requirement to allow patch updates on the **gapitoken** library (`~0.1.2`)
 - I have expanded the jwt example substantially to provide more details about its usage

I hope I have not forgotten anything. Let me know if any updates are necessary before merge.