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.
Yes, you are completely right and i would like it to be added as well. I was thinking of how to add that later. In this case i did not want to modify to much in case something would break. The logic for this would however not be that hard to add i believe.