LiPo battery low voltage alert
The Atmega328 has both Power on reset and Brown-out detection build in. You don't need to add any components. 
Read the datasheet: http://www.atmel.com/Images/Atmel-42735-8-bit-AVR-Microcontroller-ATmega328-328P_Datasheet.pdf
Read Section 15.2 and you will understand exactly what voltages will stop the microprocessor from running by activating reset.  

If you want an early voltage level detection with an indicator (while you processor can still run code), simply use the onboard A/D to provide the measurement.