Enable boost_from_average as default for binary objective (#1361)
Enable boost_from_average as default for binary objective (#1361)

* implement boost_from_average by default for binary objective

* include sigmoid_ parameter in binary regression boost_from_average

microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM