ENH: Add version command to protocol

I have a small fail2ban GUI that communicates with fail2ban via the server socket https://github.com/Sean-Der/fail2web. I would like to be able to determine the version of the server so I can then advertise what features are supported (DB support in 0.9 etc..)

Should I add a test for this and then rebase? I didn't know what level of testing is expected etc.. 

thanks!
no need to rebase -- just push a "tune up" commit dropping it