adding odumon_rtplot

odumon_rtplot is a Serial Port Real-Time Voltage Plotter for Open DC UPS Monitor,
an Arduino-based voltage monitor for unmanaged UPS devices powering
computers wit DC-DC power supplies.
You can use it to plot (battery) voltage levels using gnuplot.
It can be easily modified to plot another Arduino measurement in real time.
All measured values are accumulated in a single real-time plot and
after no more data is comming from the device,
a final snapshot in a PNG file is also produced.