MLPClassifier should support 2D arrays of string targets

[stephen-hoover]

The MLPClassifier automatically encodes single columns of string targets to integers, but does not do so with multioutput problems. It should allow users to enter an array of targets such as

y = [['spam', 'dog'], ['eggs', 'cat']]

This would make multilabel-multiclass modeling much easier, so it may require that #47 be done first.