Add require.deb.key and update deb.add_apt_key.

Now deb.add_apt_key can import keys from a keyserver
and check fingerprints. update is False.

require.deb.key adds a require-style interface for
add_apt_key.
Merge pull request #141 from smola/apt-key

Add require.deb.key and update deb.add_apt_key.