[MRG+1] TheilSen robust linear regression

A multiple linear Theil-Sen regression for the Scikit-Learn toolbox. The implementation is based on the algorithm from the paper "Theil-Sen Estimators in a Multiple Linear Regression Model" of Xin Dang, Hanxiang Peng, Xueqin Wang and Heping Zhang. It is parallelized with the help of joblib.
On a personal note, I think that the popular Theil-Sen regression would be a nice addition to Scikit-Learn.
I am looking forward to your feedback. 

Florian
Please do not store the `random_state` object as a fit attribute, just use a local variable:

```python
random_state = check_random_state(self.random_state)
```

`numpy.random.RandomState` are big object when pickled and we don't need it at prediction time, so no need to store it.