Added a targetTransform API to Picasso.

Target transform allows injecting a custom Drawable implementation into PicassoDrawable instead of BitmapDrawable. This allows developers to have direct access to bitmap and use it as a BitmapShader or apply effects to it without any need to create temporary or extra bitmaps.
    
As one possible use case we can draw bitmap as a BitmapShader on any shape, e.g. circle.
    
I have added a sample that demonstrates adding circles, sepia effect and gradient overlay over bitmaps without any extra bitmaps whatsoever.
![device-2014-01-09-171025](https://f.cloud.github.com/assets/1816227/1879943/77aaba78-7951-11e3-903c-b036cb956e5a.png)