exiv2 - add universal build option

I've been trying to use [exiv2node](https://github.com/dberesford/exiv2node) with [node-webkit](https://github.com/rogerwang/node-webkit). Annoyingly Chromium is still building as i386 so to get the linking all sorted out I needed a universal build of exiv2. 

I looked at a couple other formula and tried to follow the pattern, but let me know if there's anything that needs to be changed.