Added onAdjust Callback

The Callback parameter will be run when Anystretch is called directly.  I needed a callback that would run anytime _adjustBG completed for a project that I am working on.

I wanted to make sure my code wouldn't break any existing implementations of the plugin so I added an option called "onAdjust" to handle this.

Let me know if you would like me to change how I've implemented this solution.