Add Setuptools integration issue #261

These changes enable any user that installs `isort` to check his source
code from setup.py with the command `python setup.py isort`

I also checked isort package code with his command in tox and travis.

Acknowledgements: this is mostly inspired by flake8 setuptools integration,
see [flake8 doc](http://flake8.readthedocs.org/en/2.2.3/setuptools.html)