Doxygen Cleanup: NAT nomenclature

Hello,

I've completed a first pass through the Doxygen comments and cleaned up 
a lot of it. Those tasks aren't done (yet, I'll bring it up on Mantis), 
but I've resumed organising the Doxygen groups. I'm a little confused 
about the group with the internal name "nat", long name "NAT testing 
library" or "NAT library".

The headers that define it are "gnunet_nat_service.h" and 
"gnunet_nat_auto_service.h", which seems to contradict the two group 
names. Is NAT a service or a library?

I'm not sure which of the two long names is more fitting, as the call 
graphs and description don't imply that it is simply for testing. Which 
long name is more appropriate?

	- Willow