Major Description
Finding frequent items with possibly false positives, using the algorithm described in http://www.cs.umd.edu/~samir/498/karp.pdf.
df.stat.freqItems(cols: Array[String], support: Double = 0.001): DataFrame
The output is a local DataFrame having the input column names. In the first version, we will implement the single pass algorithm that may return false positives, but no false negatives.
