Picasso加载圆形图片和圆角图片

（1）加载圆形图片

https://gist.github.com/julianshen/5829333

改进版支持Palltte

https://gist.github.com/dtygel/71b77481ff9ddec26646

Picasso.with(this).load(url).transform(new CircleTransform()).into(ivAvatar);  

（2）加载圆角图片

https://gist.github.com/aprock/6213395

Picasso.with(this).load(url).transform(new RoundedTransformation()).into(ivAvatar);