[SPARK-5678] Convert DataFrame to pandas.DataFrame and Series

```
pyspark.sql.DataFrame.to_pandas = to_pandas(self) unbound pyspark.sql.DataFrame method
    Collect all the rows and return a `pandas.DataFrame`.

    >>> df.to_pandas()  # doctest: +SKIP
       age   name
    0    2  Alice
    1    5    Bob

pyspark.sql.Column.to_pandas = to_pandas(self) unbound pyspark.sql.Column method
    Return a pandas.Series from the column

    >>> df.age.to_pandas()  # doctest: +SKIP
    0    2
    1    5
    dtype: int64
```

Not tests by jenkins (they depends on pandas)

Author: Davies Liu <davies@databricks.com>

Closes #4476 from davies/to_pandas and squashes the following commits:

6276fb6 [Davies Liu] Convert DataFrame to pandas.DataFrame and Series