Ignore duplicate meta data values. Fixes #232

There's no need for duplicate meta data item. 
I'm considering an item duplicate when _both key and value are exactly the same_.

Doing this in `parseMeta` saves us using [`_.uniq`](http://underscorejs.org/#uniq) for every instance used in the controllers. 
e.g. for `homepageURL` or any other meta data:
```javascript
_.uniq(script.meta.homepageURL).forEach(function(homepage) {
  ....
});
```

Fixes https://github.com/OpenUserJs/OpenUserJS.org/issues/232