Make absolute URLs for success and cancelled relative too

This module was exactly what I was looking for, so thanks for developing it! One thing that struck me, though, when I was doing the configuration is that having the success and cancel URLs absolute was going to be a configuration issue in terms of having to remember to change them when a site moved from test to production. While it may not be a 'big deal', it could be an embarrassment (sending customers to a test site after they make a purchase) that I figured was easily avoidable by allowing relative URLs, and taking care of making them absolute in the code. After all, I hate doing anything a computer can do for me. :-)

So, the attached patch does just that. A function call takes care of making the URLs absolute if they aren't already, which is called (I believe) everywhere just before absolute URLs are needed, with the originally entered URLs stored in the settings. I also added some descriptions to the forms for these two configuration settings, as it wasn't clear to me right off the bat what they were for, and I needed to explain now that relative URLs were ok too.

Hope this helps others. Thanks for the great module! Would be great to get this in the next version.

Shawn