Better way to enable universe and multiverse

Instead of copying over a hardcoded file we just uncomment the correct lines via two simple sed commands.

Conflicts:
	image/sources.list
Upgrade from Precise to Trusty

Conflicts:
	image/sources.list
Migrate to Python 3

Ubuntu Trusty ships Python 3 by default, but not Python 2. Older versions already ship Python 3, so this change should be safe.
Remove /dev/log if it's a file a file.

Fixes https://github.com/phusion/baseimage-docker/pull/25
Modify defaults for syslog_ng to function properly

On 14.04 (trusty) there is nos no need to list all the modules since that was fixed.
No only that, listing them causes an error and it doesn't start.
modified setuser to use Python3 (default on 14.04)