Change PicassoDrawable to respect bounds given by its container

This series fixes issues with PicassoDrawable that cause, among other things, the ScaleType attribute of the containing ImageView to not function properly.

This should fix issues #73 and #103.

You may have other plans for fixing/changing PicassoDrawable, but this series might at least be helpful to anyone who needs  a temporary fix before the final 2.0 release.