have a working pystan

basic test:

https://stackoverflow.com/questions/56701359/running-setup-py-install-for-fbprophet-error
````
import pystan
model_code = 'parameters {real y;} model {y ~ normal(0,1);}'
model = pystan.StanModel(model_code=model_code)
y = model.sampling().extract()['y']
y.mean()  # with luck the result will be near 0
````
