[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
> handling the 1d case explicitely in _spatial_median since it speeds up by a factor of almost 9 but actually not much in an absolute way. So this could be reconsidered maybe.

+1 for keeping the 1D special case. For 1e6 or more samples it's likely to be more significant.