Support tiled Esri MapServer with limited extent

`ArcGisMapServerImageryProvider` could previously fail (cause a blank globe) when accessing a tiled MapServer with a limited extent.  While a tiled MapServer always (in my experience at least) has a level zero tile, Cesium may decide to map the level zero terrain tiles to level 1 imagery tiles.  This is common when the imagery has a `WebMercatorTilingScheme`.  Since all of the level 1 tiles don't necessarily exist, and there's no parent terrain tile to upsample from, Cesium would refuse to render the level 0 terrain tiles.

This is easily fixed by making `ArcGisMapServerImageryProvider` read the `fullExtent` property in the metadata, so that Cesium knows which tiles to expect.

To test this, add `www.ga.gov.au` to your allowed hosts, and then use this example in Sandcastle:

```
require(['Cesium'], function(Cesium) {
    "use strict";

    var widget = new Cesium.CesiumWidget('cesiumContainer');
    widget.scene.globe.imageryLayers.addImageryProvider(new Cesium.ArcGisMapServerImageryProvider({
            url : 'http://www.ga.gov.au/gis/rest/services/topography/Australian_Topography_WM/MapServer',
            proxy : new Cesium.DefaultProxy('/proxy/')
        }));

    Sandcastle.finishedLoading();
});
```