Add parameter to BingMapsImageryProvider to allow changing the culture of the imagery.

As before, default value is unspecified, i.e. en-us.

To see what this does, try:

```javascript
var viewer = new Cesium.Viewer('cesiumContainer', {
    baseLayerPicker: false,
    imageryProvider : new Cesium.BingMapsImageryProvider({
        url: '//dev.virtualearth.net',
        mapStyle: Cesium.BingMapsStyle.ROAD,
        culture: 'ja-jp'
    })
});
```

I also cleaned up the BingMapsImageryProvider specs, which were kind of a mess.