Added ability to specify the host address to start the server on

I'm using salt-api as a bridge between Salt and a custom web interface. In this use case, I have no reason to make salt-api available outside of the server running salt-api and the web interface, so I've added this setting in to allow me to change the socket host to 127.0.0.1.

The default is still 0.0.0.0 (i.e. all interfaces).