support loading indicator view without much trouble

added support for UIActivityIndicatorView while the image is
downloaded, without adding extra ‘setImageWithUrl’ methods to the
UIImageView+WebCache category.
I used [this](https://github.com/mythodeia/UIActivityIndicator-for-SDWebImage) pull request and modded it a bit to support loading indicator while the image is downloading. 
Original author is [JJSaccolo](https://github.com/JJSaccolo)