brackup-verify-inventory added

[Gavin Carr]

FWIW, I've added my brackup-verify-inventory utility to subversion
now, as well as the missing size() and chunkpath() methods it requires 
in Target::Amazon. Be good to have some people try it out on their
targets if anyone's so inclined.

Note that there's still an open bug with brackup-target prune leaving 
orphaned chunks in the inventory database, so if 
brackup-verify-inventory reports you have any bad chunks in your 
inventory it would be great if you could verify whether it's correct
or not against your target.

Cheers,
Gavin