Make stroke style of the graticule configurable

This allows the user to configure a `strokeStyle` to be used for the graticule:

![styled-graticule](https://cloud.githubusercontent.com/assets/227934/3700647/affa279c-13e3-11e4-8f0a-c5b2857c4514.png)

```javascript
// Create the graticule component
var graticule = new ol.Graticule({
  // the style to use for the lines, optional.
  strokeStyle: new ol.style.Stroke({
    color: 'rgba(255,120,0,0.9)',
    width: 2,
    lineDash: [0.5, 4]
  })
});
```

See #2167.

Please review.
can this be @const ?