Python3 incompatibility

[kbo...@broadinstitute.org]

I opened an issue on bitbucket about an issue in the reformatting of sequence headers, but it looks like it's a broader issue with functions that have slightly different behavior in python3. I was able to fix my initial issue, but ran into another downstream (after the Bowtie2 step). I'm going to try running on python2 to see if the issue is on my end, but I suspect this will work. 

As a simple fix, it might be worth changing the readme to reflect that it requires python2 (rather than >=2.7, which implies python3 will work), and restrict the downloads from pypi to exclude python3.