FIX #149: Stop importing my package on `setup.py`.

* build: Extract version with execfile() on a separate `_version.py` file.
Actually, i just noticed in the example matplotlib-link you provided that the code readd `__init__.py` file, and the possibility of something breaking is more... possible, since this file is used for many other things.

And that is exactly why I placed the version-identifier in a separate `_version.py` file, with nothing else in it!