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 (should this be an actual Sandcastle example?):

```
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'
    }
}));
```