[SPARK-7320] [SQL] Add Cube / Rollup for dataframe

Add `cube` & `rollup` for DataFrame
For example:
```scala
testData.rollup($"a" + $"b", $"b").agg(sum($"a" - $"b"))
testData.cube($"a" + $"b", $"b").agg(sum($"a" - $"b"))
```

Author: Cheng Hao <hao.cheng@intel.com>

Closes #6257 from chenghao-intel/rollup and squashes the following commits:

7302319 [Cheng Hao] cancel the implicit keyword
a66e38f [Cheng Hao] remove the unnecessary code changes
a2869d4 [Cheng Hao] update the code as comments
c441777 [Cheng Hao] update the code as suggested
84c9564 [Cheng Hao] Remove the CubedData & RollupedData
279584c [Cheng Hao] hiden the CubedData & RollupedData
ef357e1 [Cheng Hao] Add Cube / Rollup for dataframe

(cherry picked from commit 09265ad7)
Signed-off-by: Cheng Lian <lian@databricks.com>