title: PySpark computing correlation
body: <p>I want to use <code>pyspark.mllib.stat.Statistics.corr</code> function to compute correlation between two columns of <code>pyspark.sql.dataframe.DataFrame</code> object. <code>corr</code> function expects to take an <code>rdd</code> of <code>Vectors</code> objects. How do I translate a column of <code>df['some_name']</code> to <code>rdd</code> of <code>Vectors.dense</code> object?</p>