add scheme explanatory note

add scheme explanatory note.

fixed play bug and add manage cache API.

1.fixed maybe cannot play those videos that size is little .
2.add manage cache API, contain clear all cache and clear the cache for given url.

Fixed player still playing after the showView was dealloc.

Fixed player still playing after the showView was dealloc. Use AOP observe view's dealloc, when observe the dealloc view is the showView, the player will release play resource and stop play.

Fixed block main thread in old iPhone and fix multiple download requests

1. fix block main thread in old iPhone like iPhone 5s
2. fix multiple download requests run at the same time.
3. thank for @老孟(http://www.jianshu.com/users/9f6960a40be6/timeline) help me found block bug and help me test this bug after I fix those bug.

Fixed observer play status notification bug.

Avoid notification center add self as observer again and again that maybe lead to block. thank for @菜先生 commit this question.

Commit the project.

This library provides an video player with cache support in UITableView.

• Cache video data at playing
• Handle play or stop video in mian-thread
• Excellent performance!
• Always play the video of the UITableViewCell in screen center when scrolling
• A guarantee that the same URL won't be downloaded several times
• A guarantee that main thread will never be blocked