Add optional tenant JWT payload field (fixes #205)

[kpshek]

Additionally, the example JWT has been improved to include the public and private keys used to both sign and verify the signature.

This fixes #205

[isaacvetter]

I was (naively) a bit surprised that the example tenant code was a guid. There may be implementations of this optional field that will be a more human-readable string, but I understand why this is the example.