Add Python setup script

Added a setup script to allow easy installation via `python setup.py install`, `pip`, and `easy_install`.

By merging this change, you should be able to submit ROPgadget to the Python Package Index (PyPI).  This is a very [straightforward process](http://peterdowns.com/posts/first-time-with-pypi.html).

Ultimately, I would like to augment pwnies' `pwntools` by using ROPgadget instead of their internal ROP generation mechanism.  This makes that easy to do.

Other notable changes:

- Renamed `ROPgadget.py` to `scripts/ropgadget` for installation purposes
- Each `__init__.py` now imports all modules and submodules, so they are accessible via `import`