[SPARK-4435] [MLlib] [PySpark] improve classification

This PR add setThrehold() and clearThreshold() for LogisticRegressionModel and SVMModel, also support RDD of vector in LogisticRegressionModel.predict(), SVNModel.predict() and NaiveBayes.predict()
Can we set default to `None` here, and set 0.5 and 0.0 in LR and SVM, respectively? It's a little strange to put 0.5 here.