Add setup.py script

I wanted to be able to include `lifelines` as a dependency in some of my own code, so I went ahead and added a `setup.py` to the existing (great!) codebase.  I also delayed the following line until `plot_lifetimes` is actually called:
```python
import matplotlib.pyplot as plt
```
The documentation lists `matplotlib` as optional (and the `setup.py` script reflects this).

I also set some metadata as best as I could guess, including a version set to `0.1.0`, and made sure to include the extra files as well (datasets/*, Documentation.ipynb, etc.).

Let me know if anything should be changed.  Thanks,
-KS