Implemented HashedMutlilabelModel for feature hashing

This should be the default, so nobody has to init them it explicitly:
```
ret->psi_computed_sparse = false;
ret->psi_pred_sparse = SGSparseVector<float64_t>(0);
ret->psi_truth_sparse = SGSparseVector<float64_t>(0);
ret->psi_computed = false;
ret->psi_pred = SGSparseVector<float64_t>(0);
ret->psi_truth = SGSparseVector<float64_t>(0);
```