Allow fetch() to be stubbed for the RtsClient

- so that we can write some tests for it.

src/index.js

@@ -15,6 +15,7 @@ limitations under the License.
 */
 
 import Skinner from './Skinner';
+import RtsClient from './RtsClient';
 
 module.exports.loadSkin = function(skinObject) {
     Skinner.load(skinObject);
@@ -27,3 +28,7 @@ module.exports.resetSkin = function() {
 module.exports.getComponent = function(componentName) {
     return Skinner.getComponent(componentName);
 };
+
+module.exports.setFetch = function(fetchFunction) {
+    RtsClient.setFetch(fetchFunction);
+};