[Nitro] Og::Revisable implementation

[George Moschovitis]

Hello Dirk,

> in the attachment you'll find a implementation of Og::Revisable.

thanks for the patch, but while reviewing this for inclusion, I
decided to recode it ;-)
I will commit my version for the others to investigate... I think it is better.
My version does not allow to specify exact properties to keep as
revised, but will probably add this later.

regards,
George.


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