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.
I apologize for being OCD but my comments:

1. Why don't you just declare the variable as all other variables are declared above and have a var flag instead? If you do make this change, you should append this as the last entry in both logical blocks (var defs and flag defs), (if generateDir var was above noBrowser var it would be perfect already :( )
2. All other flags have full names, so perhaps use a full name '"gui-address"
3. GUI not Gui

This way the code is atleast more or less consistent.