ed25519, golang 1.7

This release includes support for the ed25519 curve (it was added to golang in 1.6). And now that 1.7 is out we're building against golang 1.7.

If you were using auto signing in a previous release the log messages were broken, they were logging the CA key fingerprint, not the user fingerprint. This has been fixed.

This has been pushed to Docker hub as cloudtools/ssh-cert-authority:v1.4.