Add the ability to parse CEL version 4 files from Affy

Hey,

I noticed that Biopython was missing the ability to parse binary CEL files (version 4), so I've added a rough implementation. I've kept TODOs in the code and a main method to demonstrate example use. I realize these are not best practices for a mature library, but this corner of Biopython (the Affy module) seems quite immature, so I figured I'd leave the code in this state to indicate to others that there is much room for improvement.

I have not contributed to this project before, so please let me know how to get this pull request in shape for a commit.

Thanks,
Jeff