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!
I actually just pulled the `"Fail2Ban v" + response` out of fail2ban-client, and was just trying to stay consistent.

I agree that the extra "Fail2Ban v" is not really needed, want me to rebase and drop it?