Add table view controller for feed item list.
1. add get data method to DZCache.
2. add class for feed header and item cells.
3. add feed view controller for feed item list.
4. modify file stream support seek beyond download data.
5. initialize play view controller with feed item.
[REMARK]
1. navigation controller always create new VC when segue triggered.
2. navigation controller always dealloc VC when navigating back dismisses the VC.
3. play view controller currently does not dealloc player; playing will continue.
[TODO]
 -  player shall be singleton.