iperf3

This is a major version update of iperf. iperf is a client-server program (in a single binary) and versions 2 and 3 are incompatible. The new binary is called ```iperf3```. I haven't used previous versions and don't know what the old binary is called. I think it makes sense to have both versions in homebrew because users might need to communicate with an older server. Debian is currently on 2.x. I don't know if the old formula should be renamed to 'iperf2' or if this one should be called just 'iperf'. Comments welcome.