Add random dataframe creation for unit tests

Usage

def test_something(self):
   df = self.randomDF()
   df = self.randomDF("a", "b")
   self.assertEqual(["a", "b"], df.colums)
   df = self.randomDF("a:bigint")
   self.assertEqual(LongType(), df.fields[0].dataType)

First stable release

Making pyspark unit tests easy and fast