Revert "remove deprecated option token_format"

This reverts commit 3d872f6739ea2973cf1928e4b9c09a926c298155.
Revert "commenting deprecated parameter token_format"

This reverts commit f31fb0e622e610a73ec02c10a5b41d28d1b9f2bd.
Merge pull request #148 from vuntz/revert-token_signing

Revert commits breaking the choice of token format