Run cython via setup.py

At the moment there's a one-line bash script [genc](https://github.com/iminuit/iminuit/blob/master/genc) that has to be used to cythonise `_libiminuit.pyx`:
```
cd iminuit; cython -a --cplus --fast-fail --line-directives _libiminuit.pyx ;
```

All other projects do this via `setup.py` and I think we should to (instead of documenting that devs have to run `genc`).
Most scientific Python projects do this ... maybe there's a simple solution that we can cut & paste into `setup.py`?