added setShadowDrawable method

Although a sample code uses `setShadowDrawable` in README, it seems not to be implemented yet.
So please let me send a PR to add `setShadowDrawable` method.

https://github.com/emilsjolander/StickyScrollViewItems/blob/21e89abc441c752b06894bde1efc7c6ac11b9ad4/README.md#L75

If any, please feel free to let me know.