We added a bunch of higher order statistics such as skewness and kurtosis to GroupedData. I don't think they are common enough to justify being listed, since users can always use the normal statistics aggregate functions.

That is to say, after this change, we won't support
```scala
df.groupBy("key").kurtosis("colA", "colB")
```

However, we will still support
```scala
df.groupBy("key").agg(kurtosis(col("colA")), kurtosis(col("colB")))
```

Author: Reynold Xin <rxin@databricks.com>

Closes #9446 from rxin/SPARK-11489.

Add Python API for stddev/stddev_pop/stddev_samp/variance/var_pop/var_samp/skewness/kurtosis

Author: Davies Liu <davies@databricks.com>

Closes #9424 from davies/py_var.

cc mengxr

Author: Davies Liu <davies@databricks.com>

Closes #8657 from davies/move_since.

Our current BinaryExpression abstract class is not for generic binary expressions, i.e. it requires left/right children to have the same type. However, due to its name, contributors build new binary expressions that don't have that assumption (e.g. Sha) and still extend BinaryExpression.

This patch creates a new BinaryOperator abstract class, and update the analyzer o only apply type casting rule there. This patch also adds the notion of "prettyName" to expressions, which defines the user-facing name for the expression.

Author: Reynold Xin <rxin@databricks.com>

Closes #7174 from rxin/binary-opterator and squashes the following commits:

f31900d [Reynold Xin] [SPARK-8770][SQL] Create BinaryOperator abstract class.
fceb216 [Reynold Xin] Merge branch 'master' of github.com:apache/spark into binary-opterator
d8518cf [Reynold Xin] Updated Python tests.

1. ntile should take an integer as parameter.
2. Added Python API (based on #6364)
3. Update documentation of various DataFrame Python functions.

Author: Davies Liu <davies@databricks.com>
Author: Reynold Xin <rxin@databricks.com>

Closes #6374 from rxin/window-final and squashes the following commits:

69004c7 [Reynold Xin] Style fix.
288cea9 [Reynold Xin] Update documentaiton.
7cb8985 [Reynold Xin] Merge pull request #6364 from davies/window
66092b4 [Davies Liu] update docs
ed73cb4 [Reynold Xin] [SPARK-7322][SQL] Improve DataFrame window function documentation.
ef55132 [Davies Liu] Merge branch 'master' of github.com:apache/spark into window4
8936ade [Davies Liu] fix maxint in python 3
2649358 [Davies Liu] update docs
778e2c0 [Davies Liu] SPARK-7836 and SPARK-7822: Python API of window functions

Add version info for public Python SQL API.

cc rxin

Author: Davies Liu <davies@databricks.com>

Closes #6295 from davies/versions and squashes the following commits:

cfd91e6 [Davies Liu] add more version for DataFrame API
600834d [Davies Liu] add version to SQL API docs

dataframe.py is splited into column.py, group.py and dataframe.py:
```
   360 column.py
  1223 dataframe.py
   183 group.py
```

Author: Davies Liu <davies@databricks.com>

Closes #6201 from davies/split_df and squashes the following commits:

fc8f5ab [Davies Liu] split dataframe.py into multiple files