New Plugin SML: Read data from a smart power meter device

This plugin read data from a smart power meter (eHz) using a network or a serial connection to the device. It tries to read the data from the IR interface on the front of the device using [SML](http://wiki.volkszaehler.org/software/sml) (Smart Message Language) protocol.

It was tested with a eHz device from Hager (EHZ363Z5 & EHZ363W5) reporting total consumption, current consumtion for both incoming (consumtion) and outgoing (delivery) power.