Pyathena

* Fix parameter format of Decimal type.
Thanks EdwardJRoss. (laughingman7743/PyAthena/issues/121)

* Fix PandasCursor to not ignore rows containing only null values.
Thanks EdwardJRoss. (laughingman7743/PyAthena/issues/117)
* Support utility queries in PandaCursor
Thanks radcheb. (laughingman7743/PyAthena/issues/114)
* Implement to_sql method in utility package.

* Support pandas.to_sql.
See [README](https://github.com/laughingman7743/PyAthena#to-sql) for details.
Thanks kismsu, DyfanJones. (laughingman7743/PyAthena/issues/101, laughingman7743/PyAthena/pull/110)
* Support executemany method with Cursor and PandasCursor.

* Redesign Formatter and Converter classes.
You can now customize dtypes and converters for the Dataframe object.
See [README](https://github.com/laughingman7743/PyAthena/blob/master/README.rst#pandascursor) for details.
* Support Bool column NA value in PandasCursor.
Thanks tanguycdls, mzhang13. (laughingman7743/PyAthena/issues/100, laughingman7743/PyAthena/issues/102, laughingman7743/PyAthena/pull/103)
* Change s3_staging_dir argument to optional.
Thanks nitin-kakkar. (laughingman7743/PyAthena/issues/99)

* Implement query result caching.
See [README](https://github.com/laughingman7743/PyAthena/blob/master/README.rst#quickly-re-run-queries) for details.
> You can attempt to re-use the results from a previously run query to help save time and money in the cases where your underlying data isn't changing.
> Set the cache_size parameter of cursor.execute() to a number larger than 0 to enable cacheing. (Cacheing is turned off by default.)

Thanks kbrose. (laughingman7743/PyAthena/pull/96)

* Check that column_default is not nan.
Thanks capitancambio. (laughingman7743/PyAthena/pull/94)