Consider these two items (or something similar):
[h=1]Anker® 2nd Gen Astro E3 10000mAh Dual-Port Ultra Compact External Battery Portable USB Charger Power Bank - I have something similar to this (actually 2) and it works great.[/h]EasyAcc® 20W 4A 4-Port USB Wall Charger with Folding Plug Portable Travel Charger
I also have something like this for general travel purposes, not just bike touring.

-or- you could go the expensive way and look into a front wheel dynamo setup with a usb outlet on the stem liek this: LightCharge Hub USB Shimano/SP