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.
> 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 :( )

I have modified the code for consistency and the variable is now moved to the var section in the code.  I will get back to cosmetics when the functionality for the PR is implemented on my side.

>2.All other flags have full names, so perhaps use a full name '"gui-address"

Fixed.

> 3.GUI not Gui.

Fixed.

I appreciate all your comments and suggestions.

> 4.Perhaps explain that it's an 'override' option if you decide to go the way calmh suggested.

Added in the comment field for flag.