GeoJson fixes and cleanup

[mramato]

1. Automatically set perPositionHeight to true if a polygon contains altitude values.
2. Remove sourceUri parameter from the geometry functions since they are no longer used (and haven't been for a while).
3. Move polygon creation code into a helper function to de-duplicate code that was in both processPolygon and processMultiPolygon
4. Added specs and updated CHANGES.

[mramato]

@emackey as we discussed offline, here's a simple example that showed the problem. In master, the polygon is drawn on the ground, even though the input provides heights. This branch correctly draws it at altitude.

//Create the viewer 
var viewer = new Cesium.Viewer('cesiumContainer');
Cesium.viewerEntityMixin(viewer);
var geo = {
    "type" : "FeatureCollection",
    "features" : [{
        "type" : "Feature",
        "geometry" : {
            "type" : "Point",
            "coordinates" : [102.0, 0.6, 1e5]
        }
    }, {
        "type" : "Feature",
        "geometry" : {
            "type" : "LineString",
            "coordinates" : [[102.0, 0.0, 1e5], [103.0, 1.0, 1e3], [104.0, 0.0, 1e5], [105.0, 1.0, 1e2]]
        }
    }, {
        "type" : "Feature",
        "geometry" : {
            "type" : "Polygon",
            "coordinates" : [[[100.0, 0.0, 1e6], [101.0, 0.0, 1e6], [101.0, 1.0, 1e6], [100.0, 1.0, 1e6], [100.0, 0.0, 1e6]]]
        }
    }]
};

var dataSource = new Cesium.GeoJsonDataSource();
viewer.dataSources.add(dataSource);
dataSource.load(geo);

//Reset the scene when switching demos. 
Sandcastle.reset = function() {
    var camera = viewer.scene.camera;
    camera.lookAt(Cesium.Cartesian3.fromDegrees(100, 0, 5000000), Cesium.Cartesian3.fromDegrees(100, 0, 0), Cesium.Cartesian3.UNIT_Z);
};

[emackey]

Looks good. Tests pass. Merging.