Added logos and included in the readme

-SDWebImage
-==========
+<p align="center" >
+  <img src="SDWebImage_logo.png" title="SDWebImage logo" float=left>
+</p>
+
+
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 [![Pod Version](http://img.shields.io/cocoapods/v/SDWebImage.svg?style=flat)](http://cocoadocs.org/docsets/SDWebImage/)
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