Loading Indicator view with 2 lines of code

added support for UIActivityIndicatorView while the image is
downloaded, without adding extra ‘setImageWithUrl’ methods to the
UIImageView+WebCache category.
I used [this pull
request](https://github.com/mythodeia/UIActivityIndicator-for-SDWebImage
), 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];
```