Support binding to a client port in addition to a client address.

[scottmsilver]

I'd like speedtest to be able to bind to a client port, in addition to an IP. I extended source_address to support IP:PORT scheme and made it so you could optionally include the port or the IP address. The reason I want to bind to a client port is that I want my firewall to be route the speedtest through different external gateways based on the client port. This allows me to run speed tests against the different egress routes on my network but run the speedtest from a device that is not my router. This should be entirely backwards compatible.