Update tfidf.js

Fixing problem when deriving tfidf for the term 'constructor' https://github.com/NaturalNode/natural/issues/119
Merge branch 'master' of https://github.com/fergiemcdowall/natural into fergiemcdowall-master
fixed #119 and #120, added unit test
Merge branch 'master' of https://github.com/NaturalNode/natural