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.

This will ignore meta data items like
```
// @description value
// @description value
```
But also (see difference in `s` and `c`):
```
// @lisense    MIT
// @license    MIT
```


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 original bug report in https://github.com/OpenUserJs/OpenUserJS.org/issues/232