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];
```