Let contentViewController decide autorotation

Currently, if MVYSideMenuController is the root controller, the default is used: UIInterfaceOrientationMaskAllButUpsideDown on iPhone and UIInterfaceOrientationMaskAll on iPad.

That's fine, but it would be nice to allow the contentViewController to determine what the supported orientations are.

The example program still rotates to all orientations, because the `contentViewController` is a UINavigationController, which returns the default values. However, if you change it to be just the MVYContentViewController, and add the `-supportedInterfaceOrientations`, you can see that it works.