Fix GPG Key Problems on Debian/Ubuntu Linux

Symptom:
W: GPG error: http://example.org unstable/ Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXX

Solution:
sudo gpg --keyserver hkp://wwwkeys.eu.pgp.net --recv-keys XXXXXXXX
sudo gpg -a --export XXXXXXXX | sudo apt-key add -

Other Key Servers:
1. pgpkeys.mit.edu
2. wwwkeys.pgp.net
3. PGP Key Server List