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, original author is [JJSaccolo](https://github.com/JJSaccolo),  and modded it a bit to support loading indicator while the image is downloading. 

the usage is basically this right before the usual sd_setImage... methods:
```
        // show activity indicator
        [cell.imageView setShowActivityIndicatorView:YES];
        // choose indicator style
        [cell.imageView setIndicatorStyle:UIActivityIndicatorViewStyleGray];
```