created the df function

- Currently unimplemented
implement dfPromise to get df for a single word
Implement TF*IDF for array of words

- analyze() given an array of words, calculates the term frequency
- then feeds that information to the tfidf function that creates an
array of promises to get the document frequency for each term
- tfidf method returns a master promise that resolves once all requests
are done
- the tfidf method must be called in conjunction with '.then"
i.e. this.tfidf.then(…)
Merge branch 'document-frequency' into tfidf