FIX Theil-Sen unittest for older Scipy Version

Usage of fmin_bfgs instead of minimize.
FIX that some functions in Theil-Sen were public

- _spatial_median, _breakdown_point and _modweiszfeld_step are now
private as suggested by agramfort.
- moved _lse before TheilSen class.
FIX usage of linalg from Scipy in Theil-Sen

Replaced usage of numpy.linalg by scipy.linalg as suggested by
agramfort.
FIX: Let Theil-Sen handle n_samples < n_features case

Theil-Sen will fall back to Least Squares like if number of samples is
smaller than the number of features.
FIX: Python 2.6 format syntax in Theil-Sen

Fix of "zero length field name in format" ValueError due to Python 2.6
in TheilSen class.
Vectorization of theilsen._modweiszfeld_step

Replaced the for-loop in linear_model.theilsen._modweiszfeld_step by
array operations as suggested by jnothman
FIX: Parallel unittests for Theil-Sen estimator

Instead of testing the various codepaths with n_jobs of joblib, now the
functions _get_n_jobs and _split_indices of the theilsen class are
tested as pointed out by agramfort.