Support for ASEM UPS on Linux/i2c

Patch from http://article.gmane.org/gmane.comp.monitoring.nut.devel/6723

Thread: http://news.gmane.org/find-root.php?message_id=53A83FCB.1080808%40copanitalia.com

Builds on Ubuntu 12.10 and 14.04; requires libi2c-dev
asem: documentation