Added a check for builduser to trueRoot. Fixes #245

I did some limited testing of this, namely:
* log in as root and issue `aura -A winetricks-git` and `aura -A --builduser=aspidites winetricks-git`
* repeat as normal user using `sudo` and `su`

It's rather early for me, so I'm not sure if I missed the existence of an actual test suite. I'd also appreciate feedback on coding style, as the conventions used in aura are somewhat different than what I've used, so I suspect that while the indentation that I used for the if expression is valid, it's not entirely in keeping with aura's overall style.