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 xtra bitmaps whatsoever.