Unwrap StickyListHeaders item before use

Fix for https://github.com/nhaarman/ListViewAnimations/issues/240

1) Fixes the getChildCount() method from StickyListHeadersListViewWrapper 
```java
@Override
public int getChildCount() {
    return mListView.getListChildCount();
}
```

2) Unwraps the item from WrapperView before trying to find a ListViewAnimations' tags from it:
```java
public View unwrapItemView(final View view) {
	if (view instanceof WrapperView) {
		return ((WrapperView) view).getItem();
	}
	return view;
}

@Override
public View getChildAt(int index) {
	return unwrapItemView(super.getChildAt(index));
}
```