[ghns] Spec v0.5

[Josef Spillner]

Hi,

as part of the GHNS revival, I've updated the spec to account for some changes 
which I thought were overdue. Two tags were removed for simplification, some 
others were added for new features.

http://ghns.freedesktop.org/spec/ghns-spec.html

All the DTDs were replaced with XSD files which will (hopefully) lead to 
better validation of the XML produced and consumed by implementations.

Josef