Support __iter__() for DataFrame and explicitly disable for Series and Index.

[ueshin]

DataFrame.__iter__() returns an iterator of columns.

And explicitly disable __iter__() with a proper error message for Series and Index.

>>> list(ks.Series([1,2,3]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ueshin/workspace/databricks-koalas/master/databricks/koalas/series.py", line 3131, in __iter__
    return _MissingPandasLikeSeries.__iter__(self)
  File "/Users/ueshin/workspace/databricks-koalas/master/databricks/koalas/missing/__init__.py", line 24, in unsupported_function
    reason=reason)
databricks.koalas.exceptions.PandasNotImplementedError: The method `pd.Series.__iter__()` is not implemented. If you want to collect your data as an NumPy array, use 'to_numpy()' instead.

Resolves #555.

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• Co-change Alert: You modified frame.py. Often test_dataframe.py (koalas/tests) is modified at the same time.

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