Add option gui-addr for dynamic gui address and port

This PR adds a `-gui-addr` option which allows you to dynamically specify address and port for the GUI interface. The setting defaults to 127.0.0.1:8080 and will only be set the first time syncthing is launched. In other words it does not modify your config.xml after initial launch. This closes #505.
This might still get us a GUI on port 34398 if the user said `-gui-addr 127.0.0.1:8080` and port 8080 was taken. I would expect it to be a fatal error if the port cannot be used as given by the user?