Added MultilabelCLRModel class

I suggest something different here.  First create a bunch of label vectors:
* labels_nil = {}
* labels_23 = {2,3}
* labels_34 = {3,4}
* labels_1234 = {1,2,3,4}

And then just do all the loss-computations in one place:
```
EXPECT_EQ(xxx, model->delta_loss(slabel_12, slabel_23));
EXPECT_EQ(xxx, model->delta_loss(slabel_23, slabel_12));
...
```

This would make the tests more readable.