'only' keyword is ignored 

``` javascript
var mq = require('css-mediaquery');
mq.parse('only screen and (min-width: 950px)')
```

``` json
[{  
    "inverse": false,
    "type": "screen",
    "expressions": [{  
        "modifier": "min",
        "feature": "width",
        "value": "950px"
    }]
}]
```

which means the 'only' clause is just ignored, so data is lost and mediaquery cannot be rebuilt (related to #13) 
