Add TfIdf.setTokenizer support

Allows defining a customer Tokenizer for TfIdf other than the default tokenizer. Useful when using other languages or wanting to tokenize input differently.
Merge remote-tracking branch 'naturalnode/master' into tfidf-tokenizer

Conflicts:
	spec/tfidf_spec.js
Merge pull request #182 from nategood/tfidf-tokenizer

Add TfIdf.setTokenizer support