Fixed the definitions for local and sync storage.

reference: [Chrome StorageArea Documentation](http://developer.chrome.com/extensions/storage.html#type-StorageArea)
```Local``` and ```Sync``` are both types of ```StorageArea```. You could think of ```StorageArea``` as an abstract class of sorts - they use it as reference in their docs, but really you're supposed to replace ```StorageArea``` with ```local``` or ```sync```. I reflected this in the definition file by having ```Local``` and ```Sync``` extend ```StorageArea```.