* solve gpg error (from {{http://en.kioskea.net/faq/809-debian-apt-get-no-pubkey-gpg-error|here}}):

get the key:
<code>gpg --keyserver pgpkeys.mit.edu --recv-key 9F0FE587374BBE81</code>
add the key:
<code>gpg --armor --export 9F0FE587374BBE81 | sudo apt-key add -</code>