Add support for picking rasterized features from imagery layers

When you extend `Viewer` with `viewerEntityMixin`, and click on an entity from CZML, GeoJSON, etc., Cesium shows the entity's properties in an `InfoBox`.  This change extends that paradigm to work with rasterized features from imagery layers as well.  So, when you add a WMS layer and click anywhere within the layer's rectangle, Cesium now does a `GetFeatureInfo` request to the WMS server.  If the request comes back with a feature, its properties are displayed in the `InfoBox`.

Currently only `WebMapServiceImageryProvider` implements the new, optional, `pickFeatures` function that enables this to work, but other providers could gain the capability as it makes sense.

Here's some Sandcastle code to try it out (look for data in Australia):

```
var viewer = new Cesium.Viewer('cesiumContainer');
viewer.extend(Cesium.viewerEntityMixin);

var imageryLayers = viewer.scene.imageryLayers;
imageryLayers.addImageryProvider(new Cesium.WebMapServiceImageryProvider({
    url : 'http://geoserver.research.nicta.com.au/geotopo_250k/ows',
    layers : 'bores',
    parameters : {
        transparent : true,
        format : 'image/png'
    }
}));
```

Should this be an actual Sandcastle example?
I don't think we need this `innerText` fallback, do we?  `textContent` works everywhere IE9+.