Fix Table View Can't Refresh Bug
Refactor Song Table View Data Source

Split `SongTableDataDelegate` to 2 separate class: `ArrayDataSource` and `Song`. Class `ArrayDataSource` handle only about table view data source, while class `Song` handle about songs' getting, refreshing and so on.
Rename `- [Song getSongsWithCompletionHandler]` to `-[Song refreshWithCompletionBlock]`
Rename `- [ImageStore loadImageByURLString:completionHandler:]` to `- [ImageStore loadImageByURLString:completionBlock:]`
Refactor File Structures
Reformating