Moved version string to `folium/__init__.py`.

This PR makes it possible check the module version via:

```python
import folium
print9folium.__version)
```

That is important for projects that needs to freeze or check what version is being used.

@birdage Care to review?  That is my last commit before the new release!

PS: It also pep8'd `setup.py` and added python 3 to the classifiers.