Upgrade to Trusty + Python3

Second attempt. Again, applied mpeterson's patch which elegantly enables uni/multiverse and updated Dockerfile to 14.04. Additionally, changed shebang lines in my_init and setuser to use python3 (new default in Trusty), and removed unused, deprecated posixfile import.

Smoke tested locally and pushed to docker index at https://index.docker.io/u/christianromney/baseimage/ for broader testing and acceptance.