Adding support for Docker containers by adding run_as_user

  * https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=514214
  * https://github.com/dotcloud/docker/issues/1951

dnsmasq tries to drop privileges. Setting user=<user> fixes this but is not
something you want as default obviously.
Merge pull request #18 from denmat/AddUserParam

Adding support for Docker containers by adding run_as_user