Implemented HashedMutlilabelModel for feature hashing

This should stay, but be changed like this:
```
CResultSet* result = m_model->argmax(m_w, i);
+    m_model->compute_dense_vectors(ret);
new_constraint.add(result->psi_truth);
result->psi_pred.scale(-1.0);
new_constraint.add(result->psi_pred);
```