Use node-webkit to create screenshots

I ported the screenshot generation to use node-webkit instead of phantomjs for better screenshots. To test this code you must have `node-webkit` v0.10.0-rc1 (can be found [here](https://groups.google.com/forum/#!topic/node-webkit/zysKp3n37_4)) on your `PATH`. Might need to build an auto installer light phantomjs has. 

Be aware that `option.crop` isn't supported yet, so the test will fail. I have no idea on how to implement the crop functionality yet, maybe someone can help with that.  

I ported it to `node-webkit` instead of `atom-shell` because of [this](https://github.com/atom/atom-shell/issues/228#issuecomment-47221145) issue