Add Sudo user, during install, with passwordless ssh access

First of all, thanks for the awesome script! I've been wanting to jump into NGINX, and this has really helped. Hopefully I can contribute to its awesomeness.

This PR will add a new sudo user with key based SSH access. Since root login is no longer allowed after running install.sh, this solves the issue when people like me add a new user, but forget to make that new user a sudoer.

I've tested this on Ubuntu 12.04 and Debian 6.0. It's actually ported from an older server build script that I wrote at https://github.com/betweenbrain/ubuntu-web-server-build-script/blob/master/build.sh, and has been used by a few folks.

If you like this idea, what do you think about taking it one step further and disable SSH password authentication all together, like at https://github.com/betweenbrain/ubuntu-web-server-build-script/blob/master/build.sh#L140?

Thanks again!