How to monitor 12 V LiPo charging from Arduino
If you want to buy something off the shelf that will provide power for an Arduino, try a power bank - they have USB out, and are also charged via USB. I found that I had to put a 100 Ohm resistor to stop mine going to sleep, but it's +5V native.

A 100 ohm resistor (0.25 watt at least!) draws 50mA, so a 5200mAH powerbank like http://www.dx.com/p/universal-5200mah-external-li-ion-battery-charger-power-bank-w-led-indicator-usb-cable-pink-362725#.VQneCnWjlIc would leak it empty in 104 hours. If you use at least 50mA at all times, then this isn't needed - I think an Arduino draws 35mA already, so you should get away with a 300 Ohm resistor, drawing 16mA. Some experimentation might be needed.