Please add multi-class option to polyssifier

[sergeyplis]

Current exclusive support of binary classification is limiting the use of the tool. Multi-class problems arise frequently and it would be nice to be able to use polyssifier for multiclass data.

• a convenient feature would be to limit classifiers to inherently multiclass
• the default may be one vs all for all classifiers that support this
• it would be nice to be able to automatically reweigh classes for imbalanced datasets

[alvarouc]

See #18

Please try it and let me know if it works for you. After that, I can merge it to master.