Fix bug in node-webkit when requiring nw.gui

Currently the following line throws a compiler error:
```TypeScript
import gui = require("nw.gui");
```
(you get the error that the external module "nw.gui" could not be found)
Since this should work like it is documented in the docs of node-webkit, this is kind of a bug:
https://github.com/rogerwang/node-webkit/wiki/Shell

The solution was to put the module declaration in quotes. Now it compiles like a charm.

I also changed the test to do it like shown in the docs.