Working with extract-partitions.py

This pull request of on the branch I have created to work on https://github.com/ged-lab/khmer/issues/144

As mentioned in my [comment](https://github.com/ged-lab/khmer/issues/144#issuecomment-23638196), I am starting off with adding code comments.

I shall keep pushing to this branch with updates and once done, we can look at merging!