[MRG] Refactor accuracy score + Fix bugs with 1d array

This pull request intends to reduce the amount of redundant code between `accuracy_score` and `zero_one_loss` (see issue #1748). In order to achieve this,  a normalize option has been added to `accuracy_score`.

Furthermore,  mix representation of 1d vector are now properly handle by all the metrics that comparse `y_true` to `y_pred`.