Introduce ECDSA

This just adds tests; hooking this up with paramiko comes in the next
commit.
Hook up ECDSA to hostkeys

More sophisticated key negotiation is still necessary in the case
where we have an ECDSA key for the server and it offers us both RSA
and ECDSA. In this case, we will pick RSA and fail because we don't
have it. Instead, we should pick ECDSA. Still, this works if you tell
your server to only offer ECDSA keys :)