Run drone daemon as unprivileged user

Hi again :)

Running the drone daemon as root is not such a good idea, especially for a service that exposes a web front-end.
As far as I tested, drone works flawlessly as a normal user if it can access the docker socket (group docker) and has write access to /var/lib/drone.

But update will break setups that use a web front-end port under 1024, since drone cannot acquire such a privileged port. :/ 

I changed the default port to 8080 and adapted the readme, a reverse proxy is needed in front of drone to allow access to privileged ports (and imho heavily recommended for TLS). At very last I added apt-get --purge code for total cleanup.

Also tried to implement a "drop to unprivileged user" feature, but for now Go developer discourage [using it](https://code.google.com/p/go/issues/detail?id=1435#c19)

PS: I'am very sorry if you got some mails from my drone, completely forgot that the default .drone.yml includes your mail address :/ should not happen again.