rs/SDWebImage — pull request #771 by Whirlwind (2014-06-19 14:51:57+00:00)
Title: Refactor the cancel logic

I found there are many cancel function in each categories. But they are some error:

1. some cancel function have same name.
2. lost some cancel function (e.g. -[UIButton cancelBackgroundImageLoad])
3. UIButton's cancel function should have `UIControlState` argument.

So I refactor the cancel logic to easy.