[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
@arjoly Do you really think it improves code readability? My current version of `_check_subparams` has 30 lines while `fit` has 50. Following the *single level of abstraction principle* from the clean code developer movement this is already too much. This is why my first commits had even more functions. The `fit` method should be only concerned about performing the actual algorithm that calculates the parameters, not also about checking some restrictions of some hyper parameters. I just wanted to point this out, I will of course inline it if the convention is different in Scikit-Learn.