fixed npm path to main script

Your current npm version does not work as you will get:

```
Error: module "./lib-cov/properties" not found from "/Users/bodokaiser/Documents/Repositories/node-exempel/node_modules/tea-properties/index.js"
```

I removed fixed this + normalized the file structure (most libraries have there main script at `lib/index.js`).

Would be happy to have this updated quickly so I can use it for [exempel](https://github.com/bodokaiser/node-exempel).

Best,
Bo