The data source for NIPagingScrollView.
[NIPagingScrollViewDataSource] Fetching Required Album Information
< NIPagingScrollViewPage > *)
Fetches the total number of pages in the scroll view.
The value returned in this method will be cached by the scroll view until reloadData is called again.
Fetches a page that will be displayed at the given page index.
You should always try to reuse pages by calling dequeueReusablePageWithIdentifier: on the paging scroll view before allocating a new page.