SIG(0)

This implements SIG(0) signing and verification of one-off messages based on the interface outlined in your sig0 branch. Documentation could do with some work—at the least removing the brackets from the SIG(0) header so godoc displays it properly—but otherwise the basics are there. Can you take a look?
This is now in master, but still some comments :) 

Could you run over this code and check the offset+X statements and double check if you are not overrunning the msg buf?