Added comparator parameter

Added comparator parameter to ensure that the size searched for in the filter is actually the size that is returned.

Consider the following settings.
```javascript
app.value('psResponsiveConfig', {
    sizes: [{
        name: 'smallest',
        minWidth: 0
    }, {
        name: 'small',
        minWidth: 480
    }, {
        name: 'medium',
        minWidth: 720
    }, {
        name: 'large',
        minWidth: 960
    }, {
        name: 'largest',
        minWidth: 1200
    }]
});
```

The filter in `getWidthFromLabel` would actually turn two results (largest and large) if the label passed is large. Adding the comparator parameter ensures that the label searched for is an exact match instead of a loose match.