Manually display nodes

Method to force display of nodes (main-thread affined). Added a sample project that will demonstrate how to manually display nodes. Removed the UIWindow layer pool hack.
this inverts the previous behaviour of rendering subnodes before supernodes.  let's make this—

```objective-c
for (ASDisplayNode *node in self.subnodes) {
  [node recursiveDisplay];
}

[self display];
```