Added Warning Cell Voltage
Added Warning Cell Voltage to Battery voltage in Configuration tab. When battery has deployed to level that is defined with warning cell voltage buzzer starts slow beeping and copter can still land safely. When min voltage is reached beeper start fast "screaming" and LIPO is empty (to point when it could be damaged from over discharging if using that default 3.3V).
Merge pull request #68 from jaahaavi/dev

Added Warning Cell Voltage