Add Page View Controller

Let page view controller manage the mvc (and song list view, channel list view)
Make Song Class a Singleton

1. Rename class `Song` to `SongStore`
2. Modify class `MainViewController` for `SongStore` change
Add New API `- [SongStore loadWithDataRefreshBlock:completionBlock`
Update `MainViewController`

Use new API `- [SongStore loadWithDataRefreshBlock]` instead of
`- [SongStore loadWithDataRefreshBlock]`. In case of unnecessary refreshing data.
Add `SongTableViewController` to Display Songs List

1. Add `SongTableViewController` to page view controller
2. Set `MainViewController` as the initial view controller of page view controller
Select Song Table Cell Can Change Song

1. Let `PageViewController.viewControllers` public for getting `MainViewController`
2. Let `- [MainViewController changeSong]` public for changing song