get rid of n² loop in WordNet shortest_path_distance
speed up path computations in WordNet reader

1296 calls to shortest_path_distance for pairs of random nouns
used to take 6.752s, now 5.364s. Subtracting 2.187s to load NLTK
and WordNet, that's 44% off.

The main speedup comes from omitting the sort in _related.
Merge pull request #712 from larsmans/wordnet-paths

WordNet paths