[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 put a leading `_` to mark this function as private: we plan to have some utils functions officially part of the public API but rather do that progressively, only for new old functions that have been proven useful outside the project. Others will be deprecated and marked private progressively as well.

New utils should all be marked private by default.