Python 3 compatibility

[loehndorf]

The code is only compatible to Python 2.7. I would propose a pull request but could not get it to work with Python 3.

Here is a workaround for the time being:

import subprocess 
process = subprocess.Popen("python2.7 doppelganger.py",
    shell=True,stdout=subprocess.PIPE,
    stderr=subprocess.PIPE
)
output, error = process.communicate()

It is only a building block, but maybe this is useful for anybody who wants to use it from Python 3 now.