Electronics

Typical USB charging information:

• Voltage            5.00 +- 0.25 V 
• Amperage        0.5 - 0.9 A (up to 5 A for some charging units)
• Wattage           5 V * 1 A  ~  5 W

Generator:

After doing some research, I found a three phase generator that matched the USB charging power requirements. The generator is actually a hobby motor used in small RC cars. 

http://www.hobbypartz.com/26p-blcombo-toro18-25a-4300kv.html

This generator is a 14 turn 4 pole and has a 4300 KV rating. This means that for every 4300 RPM the generator is spun, it outputs 1 volt. Expected RPM for a typical wind turbine with a 15 MPH wind is ~100-300. This means that in order to achieve a 1 V output from the generator, the turbine would need to have an ~40:1 gear reduction.

A hex-bridge rectifier will convert the 3-phase alternating current into a direct current. The DC output of the rectifier will then feed a DC-DC boost converter to regulate the voltage at 5 V which will power the USB port.