Support for different tokenize functions

The user can now provide a different tokenize function. Eg: tokenize and
remove stopwords

    var normalize = function normalize(word) {
        return word.toLowerCase().replace(/['"”“\]\[<>#$%&’‘\+\*]/g,"").trim();
    };

    var stopWords = ["the"];

    var tokenize = function (text) {
	    words = text.split(/[\s\-_():.!\?,;\n]+/);

	    return words.map(exports.normalize).filter(function(word) {
                return word.length > 2 && (!~stopWords.indexOf(word)) && !word.match(/\d/g);
            });
    }

    var classifier = bayes({tokenizer: tokenize})