Add tentative support for ECDSA

This adds (incomplete) support for ECDSA using the library at https://github.com/warner/python-ecdsa . Key negotiation still needs some work -- if the server offers us RSA and ECDSA keys, we ask for the RSA key, without any consideration of what we actually have in the known_hosts file. Still, it worked for me when I turned off RSA and DSA keys in my sshd_config.