Use encoding/json for SessionState serialization (#63)

* Use encoding/json for SessionState serialization

In order to make it easier to extend in future.

* Store only email and user in cookie when cipher is unavailable

This improves safety and robustness, and also preserves the existing
behaviour.

* Add TestEncodeSessionState/TestDecodeSessionState

Use the test vectors with JSON encoding just introduced.

* Support session state encoding in older versions

* Add test cases for legacy session state strings

* Add check for wrong expiration time in session state strings

* Avoid exposing time.Time zero value when encoding session state string

* Update CHANGELOG.md