Designing 14.8v Li-ion battery charger + 5v power regulator for Raspberry Pi (of other 5v device)
You're looking for a power management IC. 

Here's an example of one

http://www.linear.com/product/LTC3553-2

it includes a LiIon charger, linear and switching regulators to charge from USB and then switch back and forth between USB and the LiIon battery. Possibly not the part you want, but it's a start.