Summary:
Simply add an `onLayout` callback to a native view component, and the callback
will be invoked with the current layout information when the view is mounted and
whenever the layout changes.

The only limitation is that scroll position and other stuff the layout system
isn't aware of is not taken into account.  This is because onLayout events
wouldn't be triggered for these changes and if they are desired they should be
tracked separately (e.g. with `onScroll`) and combined.

Also fixes some bugs with LayoutAnimation callbacks.

@public

Test Plan:
- Run new LayoutEventsExample in UIExplorer and see it work correctly.
- New integration test passes internally (IntegrationTest project seems busted).
- New jest test case passes.

{F22318433}

```
2015-05-06 15:45:05.848 [info][tid:com.facebook.React.JavaScript] "Running application "UIExplorerApp" with appParams: {"rootTag":1,"initialProps":{}}. __DEV__ === true, development-level warning are ON, performance optimizations are OFF"
2015-05-06 15:45:05.881 [info][tid:com.facebook.React.JavaScript] "received text layout event
", {"target":27,"layout":{"y":123,"x":12.5,"width":140.5,"height":18}}
2015-05-06 15:45:05.882 [info][tid:com.facebook.React.JavaScript] "received image layout event
", {"target":23,"layout":{"y":12.5,"x":122,"width":50,"height":50}}
2015-05-06 15:45:05.883 [info][tid:com.facebook.React.JavaScript] "received view layout event
", {"target":22,"layout":{"y":70.5,"x":20,"width":294,"height":204}}
2015-05-06 15:45:05.897 [info][tid:com.facebook.React.JavaScript] "received text layout event
", {"target":27,"layout":{"y":206.5,"x":12.5,"width":140.5,"height":18}}
2015-05-06 15:45:05.897 [info][tid:com.facebook.React.JavaScript] "received view layout event
", {"target":22,"layout":{"y":70.5,"x":20,"width":294,"height":287.5}}
2015-05-06 15:45:09.847 [info][tid:com.facebook.React.JavaScript] "layout animation done."
2015-05-06 15:45:09.847 [info][tid:com.facebook.React.JavaScript] "received image layout event
", {"target":23,"layout":{"y":12.5,"x":82,"width":50,"height":50}}
2015-05-06 15:45:09.848 [info][tid:com.facebook.React.JavaScript] "received view layout event
", {"target":22,"layout":{"y":110.5,"x":60,"width":214,"height":287.5}}
2015-05-06 15:45:09.862 [info][tid:com.facebook.React.JavaScript] "received text layout event
", {"target":27,"layout":{"y":206.5,"x":12.5,"width":120,"height":68}}
2015-05-06 15:45:09.863 [info][tid:com.facebook.React.JavaScript] "received image layout event
", {"target":23,"layout":{"y":12.5,"x":55,"width":50,"height":50}}
2015-05-06 15:45:09.863 [info][tid:com.facebook.React.JavaScript] "received view layout event
", {"target":22,"layout":{"y":128,"x":60,"width":160,"height":337.5}}
```