Handle ssh-rsa and ssh-dss certificate files

This does not implement certificate based authentication (described here
http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD)
but instead makes it so that if the certificate is not needed for
authentication net-ssh doesn't cause the entire application to die.

The net-ssh test suite continues to pass. On my own machine I did tests
with certificates loaded and verified that although authentication could
not proceed to a host requiring a certificate it at least did not die.

I also verified that I can continue to use normal rsa and dsa keys to
ssh to hosts that do not require certificates even when the certificates
are loaded into my ssh-agent instance.

This is a potential solution to issue #124 and an alternative to the one
presented in pull request #134.