In tree, min_samples_split and min_samples_leaf now accept float number as percentage.
TST fix precision failure on windows
MAINT 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
Change doc and add tests for random forest and gradient boosting