Revise setup.py to enable install of guessit CLI

These revisions enable **python setup.py install** to place a CLI of
**guessit** to the system's default Python CLI directory (e.g.
**/usr/local/share/python/** on OSX with Python installed via Homebrew).
As a result, a user can simply issue the command **guessit** instead of
the current **python guessit.py**.

**NOTE**: these revisions do not alter the code base itself, but instead move **guessit.py** from the root folder to **guessit/__main__.py** and enable the install of the **guessit** CLI using the**entry_points** arg in **setup.py**.