[SPARK-7509][SQL] DataFrame.drop in Python for dropping columns.

Author: Reynold Xin <rxin@databricks.com>

Closes apache#6068 from rxin/drop-column and squashes the following commits:

9d7d5ec [Reynold Xin] [SPARK-7509][SQL] DataFrame.drop in Python for dropping columns.

python/pyspark/sql/dataframe.py

@@ -1014,7 +1014,7 @@ def withColumn(self, colName, col):
 
     @ignore_unicode_prefix
     def withColumnRenamed(self, existing, new):
-        """REturns a new :class:`DataFrame` by renaming an existing column.
+        """Returns a new :class:`DataFrame` by renaming an existing column.
 
         :param existing: string, name of the existing column to rename.
         :param col: string, new name of the column.
@@ -1027,6 +1027,18 @@ def withColumnRenamed(self, existing, new):
                 for c in self.columns]
         return self.select(*cols)
 
+    @ignore_unicode_prefix
+    def drop(self, colName):
+        """Returns a new :class:`DataFrame` that drops the specified column.
+
+        :param colName: string, name of the column to drop.
+
+        >>> df.drop('age').collect()
+        [Row(name=u'Alice'), Row(name=u'Bob')]
+        """
+        jdf = self._jdf.drop(colName)
+        return DataFrame(jdf, self.sql_ctx)
+
     def toPandas(self):
         """Returns the contents of this :class:`DataFrame` as Pandas ``pandas.DataFrame``.