DOC: fix 2 covariance examples rst math markup.

Some formulas in plot_mahalanobis_distances and
plot_robust_vs_empirical_covariance were not correctly rendered by sphinx
as some characters were not properly escaped. This is fixed by setting the
docstrings as raw string using r"""

There was also missing braces for the number of features and samples subscript.
Implementation of logistic_regression_path.

This is mostly the code from Gael's logistic_cv branch. My work is only some
minor cleanup some added tests and benchmarking.
Take into account @agramfort's comments.

Only cosmetic changes.
Some fixes for LogisticCV object
Docstring of LogisticRegressionCV
Docstring
Refactor and some bug fixing
Docstring
FIX missing import
FIX: bug in LogisticRegressionCV
Make tests deterministic
FIX: coef.shape
Just to be sure
Add test
BUG when fit_intercept=False
Fallback for failing line search
Remove warning (not needed any more)
Compatibility for old scipy
iterate some more on line search
cosmetic