WIP scorers: F1 pairwise clustering metric implementation

You don't use inters variables directly, maybe it is better to write:
``` python
intersections = len(c_gold_pairs & c_system_pairs) # both are frozensets
precision = float(intersections) / len(c_system_pairs)
recall = float(intersections) / len(c_gold_pairs)
```

Also, your function `calculate_pairs` can return a frozenset.