Get version number from module attribute

Use `__version__` attribute from `__init__.py` to specify the module version number.

From Python code it is possible to get version using an import statement.

```python
from reconfigure import __version__
```

From other scripts, like `Makefile`, this can be done using a python one line command:

```
python -c "from reconfigure import __version__; print __version__"
```

See http://www.python.org/dev/peps/pep-0396/ for rationale.