Add main to package.json

Without the main property being set, this line from the readme:

```js
var DN = require('designer-news');
```

throws this error:

```
Error: Cannot find module './node_modules/designer-news'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Object.<anonymous> (/Users/christianbundy/Development/dn-test/main.js:1:72)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Function.Module.runMain (module.js:497:10)
```

I've also added a [newline at the end of the file](http://stackoverflow.com/questions/729692/why-should-files-end-with-a-newline). Please let me know if you have any questions/concerns. Cheers!