Add setup.py script for PyPI integration

Add setup.py script for PyPI integration. Excludes scripts which must be built from source.

This allows easy installation via `python setup.py install`, `pip`, and `easy_install`.

By merging this change, you should be able to submit pwntools to the Python Package Index (PyPI).  This is a very [straightforward process](http://peterdowns.com/posts/first-time-with-pypi.html).