hack of VOEvent to include W3C signature

[Matthew Graham]

Hi,

One aspect of reliable messaging is ensuring the packet comes via the 
route you think it should so if a user subscribes to a broker how can 
they verify that packets are coming via the broker and not from some 
phishing broker: only if the broker attaches its signature to the 
VOEvent as it relays packets from a publisher (who also signed the VOEvent).

I would like to second Steve's additions to v1.1 of the schema.

    Cheers,

    Matthew