Specify npm included files explictly in package.json

This just bugged me a little, figured we could save our selves a couple files (and seeing we're currently the 17th most downloaded package, a tiny bit of bandwidth xD)

```sh
$ ls node_modules/backbone/
backbone.js       bower.json      CONTRIBUTING.md  LICENSE
backbone-min.js   CNAME           favicon.ico      package.json
backbone-min.map  component.json  index.js         README.md
```

I also dropped the `index.js` in favour of a `main` param, useful mainly for browserify