[MRG] Multi-label metrics: accuracy, hamming loss and zero-one loss

This pull request intents to bring 3 new features:

- a tested and generalized  `unique_labels` function;
- multi-labels support for `accuracy_score` and `zero_one_loss` functions;
-  the hamming loss metrics (`hamming_loss` ) with multi-label support.

Before merging, I would like to suggest to add a new module where multi-labels utilities such as `unique_labels` and ` _is_label_indicator_matrix` are collected.

Furthermore, I have to re-organise (cosmit) some of the function in a multi-label categories in `metrics.py`. But I will wait that reviews are done.

This pull request also tackles issue #558. Reviews and comments are welcome! :-)