Scrolled & Fixed Descendants, Mouse Position

This is to fix these two issues:
http://groups.google.com/group/fabricjs/browse_thread/thread/abb40e65a64ecaf3?pli=1
https://github.com/kangax/fabric.js/issues/82

I edited getPointer() to consider any scrolled and/or fixed-positioned descendants. Please review to see if this is up to snuff. There are a few more details in my commit comment.

BTW, I'm new to github and git, so please let me know if I'm doing something completely wrong here. Hope I could help out.