Add Debian/Ubuntu packaging

There has been at least 4 independent attempts to package DNSCrypt for Ubuntu and provide it in PPA form, including mine. I got fed up with the massive duplication of effort and consolidated all the ideas in this packaging.

It should work for both Debian and Ubuntu, but designed with Ubuntu and its defaults in mind. It includes an Upstart job that reads configuration from /etc/default/dnscrypt-proxy file, and the AppArmor profile, improvements to which I've recently submitted.

Since Ubuntu has dnsmasq on 127.0.0.1:53, I've set DNSCrypt to run on 127.0.0.2:53 by default. I have also seen a 127.0.0.1:53000 variant, but I haven't checked if it actually works. I'm open for changing the default if anyone can offer more insight on this matter.