add an ol.layer.xyz factory

test/spec/api/layer/xyz.test.js

@@ -0,0 +1,17 @@
+describe('ol.layer.xyz', function() {
+
+    it("doesn't allow empty construction", function() {
+
+        expect(function() {
+            // nowhere
+            var layer = ol.layer.xyz();
+        }).toThrow();
+
+    });
+
+    it("creates an ol.layer.XYZ instance", function() {
+        var layer = ol.layer.xyz({url: 'http://foo/{x}/{y}/{z}'});
+        expect(layer).toBeA(ol.layer.XYZ);
+    });
+
+});