add a wallet merge tool

There is now a command to help you merge wallets together, which means you're not at risk of losing coins when deleting or moving around other wallet files. This is also useful for people mining on many computers at the same time.

Thanks to this feature, I was able to recover 4m of the 6m I lost. I now am not sure whether I actually lost 6m or just 4m. Regardless, a significant chunk is back.