TST fix precision failure on windows
MAINT skip joblib multiprocessing tests on travis
Merge pull request #3361 from ogrisel/travis-no-multiprocessing-joblib

[MRG] skip joblib multiprocessing tests on travis
FIX assert_array_almost_equal for windows tests
Cosmetic: correct latex formula display
DOC: write cost function of logistic regression.

I feel it important to state clearly what is the cost function of the
model because:

    * Some might want to compare their implementation against this one,
    in which case it is important to know exactly what is the form of the
    cost function

    * Furthermore since the models are quite inconsistent in terms of
    notation for the regularization parameter: some models in this
    module penalize by 1/C, others by alpha and others by alpha/2
Merge pull request #3365 from fabianp/master

DOC: write cost function of logistic regression
Link my name to my wordpress blog
Merge pull request #3366 from MechCoder/link-to-blog

FIX: My name was linked to the wrong github account
In tree, min_samples_split and min_samples_leaf now accept float number as percentage.
Change doc and add tests for random forest and gradient boosting