Add support for using Google Certificate Catalog as a notary

[von]

Add support for using Google Certificate Catalog as a notary: http://googleonlinesecurity.blogspot.com/2011/04/improving-ssl-certificate-security.html

Fairly straight forward with http://www.dnspython.org:

a = dns.resolver.query("405062e5befde4af97e9382af16cc87c8fb7c4e2.certs.googlednstest.com", "TXT")
for rdata in a:
    print rdata
"14867 15065 75"

[von]

Wrote a simple client whose code could be re-used: https://github.com/von/google-cert-checker