[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()
`>=` -> `>`. Because when all features are zeros, `prob = 0.5`, and in this case, it is safer to treat it as a negative.