Detecting credit card edges using opencv and python

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I'm hoping this is very simple; I suspect that's the case.

I have an app that takes an image from the user in which they're holding a credit card up, stripe side out. Something like this: http://imgur.com/OOanf9i

This is already being fed through a python script to assist in pupil detection, and I'd like to add the ability to detect the edges of the credit card, width-wise. I've done some research into openCV but it seems like a VERY in-depth topic, and I believe this is a simple case.

Can anyone give me any direction on how to accomplish this? Or just how difficult it might be to do?

Thanks!