Basic node-webkit service

Exposes ```nw.gui``` and main window.

## Example

```js
define(['./module'], function(controllers) {
    'use strict';

    controllers.controller('MyCtrl', ['$scope', 'nwService', function($scope, nwService) {

            // Minimize the main window
            nwService.window.minimize();

            // After 1 second, maximize the window
            setTimeout(function(){nwService.window.maximize();}, 1000);
            
            // When the window is closed, ask for confirmation before actually closing
            nwService.window.on('close', function() {
                if (confirm("Do you really want to close the window?"))
                    nwService.window.close(true);
            });


        }]);
});
```