Node webkit

Can you give the osx version a test before you merge this?

If you have npm installed you should be able to just go to the root of the directory and type the following commands:

```
npm install
gulp dist-node-webkit
```

It should pull down everything needed to build a node webkit app. It'll create three apps in `dist/slamdata/`: one for linux, one for osx and one for win. If the osx one works, then it should be good to merge.