Add hmac-sha2-256 to the list of supported macs

Similar to the work in #161 and #164 but simpler.

I have tested this with a demos/demo_server.py and the following ssh command line on OSX:

    ssh -l robey -o 'MACs=hmac-sha2-256' -vvv -p 2200 localhost

It connects and auths okay.

I tried to add hmac-sha2-512 as well but this has kex problems that I didn't
want to dig into here