rs/SDWebImage — pull request #988 by morizotter (2014-12-16 03:46:31+00:00)
Title: Improve operation behavior

To get more information, enhance `SDWebImageDownloaderOperation`. Especially, it is enabled to measure RTT - Round Trip Time - and sizes in this modification

## Changes made

1. Set self to all notifications.
2. Add NSURLResponse to property to get more information.
3. Declares expected size and response on header file to get information.