added idf cache to seriously speed up tfidf searches when searching large data sets, unit tests on the way
fixed a potential bug, when adding a document, cache must either be updated or invalidated.
typo in addFileSync, added cache-clearning logic to addFileSync, cleaned up a unit test
working on fixing unit test
finished unit test to ensure caching didnt break adding documents via add document
updated unit tests for tfidf to include addFileSync
Merge pull request #104 from kkoch986/master

TFIDF Performance Improvments