Added LiFePO4 batteries & set to default

I added LiFePO4 batteries to the list and set them as the default option. Upper voltage is set to 10.9 Volts (circa 3 * 3.65V) and lower to 6.6V (3 * 2.2V) (values according to http://www.powerstream.com/LLLF.htm, set the lower limit a bit higher than absolute minimum to have a healthy security gap).