Make callback URLs configurable

There are times when you might want to specify your own custom callback URLs. For instance, I am currently implementing a CTools multistep form. When I redirect to DIBS, I want it to return to the proper step in the form so that the bredcrumb and other information is kept in place. While I know this means that I have to write some code to do the work that would normally be done at that URL, its something that I need to do. I wrote a patch to do this, but it might be able to do some improvements. Let me know what you think.