add plugin to monitor "smart meters" power usage

this connects munin with the RTLAMR project, and allows readout of AMR
(Automatic Meter Reporting) data over the airwaves

This plugin will draw some nice graphs of your neighbors (and
yourself, coincidentally) power consumption. You can also see the
adoption of the AMR in your neighborhood and density of signal.
anonymise labels

data is still stored with the full identifiers so this change is
backwards compatible

but at least we are not retransmiting data off the airwaves and it is
anonymised in some way
follow file naming convention here
Merge pull request #472 from anarcat/plugin/amr

add plugin to monitor "smart meters" power usage