Added InTheaters: movie rating in query to filter results #839 

@moollaza The ia only gets triggered on single rating in query, so ive skipped the case for having more than 1 rating in query. Also, i think we should add unrated in the ratings (currently we have g, pg, pg-13, r).
I noticed this other issue, https://duckduckgo.com/?q=movies+opening (last movie)
![issue_in_theater](https://cloud.githubusercontent.com/assets/1424236/3625076/72226d72-0e65-11e4-8636-a2e16fde8f46.png)

//cc @jagtalon

@killerfish We can actually do away with all this logic and replace it with 3 lines in our `normalize` function:

```javascript
normalize: function(item){
    if ( filter_rating && item.mpaa_rating.toLowerCase() !== filter_rating) {
        return null;
    }
    ...
    return { ... };
}
```

This way when a rating is defined we can quickly check if the current item's rating matches it. If not discard that `item`.