Support of Session Serialization - update to Nimbus 1.5.3

[Bernard Wittwer]

Hi Simon,

NimbusDS 1.5.3 has been released to support serialization of srp sessions (see issue 3).

It would be great to bump to this nimbus version and also make the thinbus related classes as implementing Serializable.

Thanks, Bernard.

[simon]

pushed release 1.2.1 to sonatype will be synchronised to maven central in some hours.