Cosmits and bug fix of astype
TEST: don't use rng.choice
FIX: test_imputation_most_frequent
Cosmits in tests
PEP8/Flake8
Merge branch 'missing_values_imputation' of github.com:NicolasTr/scikit-learn into missing_values
Set missing_values=0 by default
Flake8
Use string "NaN" for imputing NaN missing values
Revert changes to base.py
Merge pull request #1 from glouppe/missing_values

Cosmits and small fixes