using setuptools instead of distutils

Note that:
* distutils doesn't suppor `intall_requires`, `extras_require`, ...
* it currenlty works with pip only because pip uses setuptools internally, but
  `python setup.py install` is broken

setup.py

 #!/usr/bin/env python
 
-from distutils.core import setup
+from setuptools import setup
 
 setup(name="sn",
       version="0.2",