Add versioning parameters to secure_cookie/signed_value APIs.
Add a v2 secure cookie format.

This format fixes some weaknesses in the original format that would allow
characters to be shifted from the "value" field to the "name" or "timestamp"
fields.  It also upgrades the signature from HMAC-SHA1 to HMAC-SHA256,
adds an explicit version field, and adds an as-yet-unused field to
support key rotation in the future.
Document the cookie versioning interface.
v3.2.1 release notes
Set version number to 3.2.1