I'm the author of this module.
At first, Yahoo API may be needed because this module uses WebIDF.
But if enough DF(Document Frequency) scores are stored, this module works well without Yahoo API.
I recommend that you set fetch_unk_word_df to false value.
(The default value is false.)
In this case, the average DF(Document Frequency) score is used.
The number of unknown word is infinity.
But the number of word(noun) which exists in the dictionary of MeCab is finite.
I'll provide bigger DF files.