Some coverage for wordnet.

Add some coverage of wordnet.

I notice that some of the callbacks in this section are in the form `callback(result);` in the successful case. In an error case, the callback is called with the error as the first argument.
I suggest changing the way the callback is called to `callback(err, result);`. In a successful case, `err` would be null. In an error case, `result` would be null.