ENH support multilabel targets in LabelEncoder

Also, support 1d-array of arrays (with the outer array having `dtype=object`) as a multilabel format and their `np.vectorize`d processing.

This makes it easy to support multilabel data with string (or non-consecutive / negative-inclusive) labels. In particular, `bincount` and related operations that can be used on LabelEncoded multiclass targets can also be used with multilabel data.

Also added inheritance of `LabelBinarizer` from `LabelEncoder`.