Pyathena

* Fix AttributeError: 'Engine' object has no attribute 'connection'.
Thanks mrshu. (laughingman7743/PyAthena/issues/63)

* Allow empty results from PandasCursor.
Thanks jkleint. (laughingman7743/PyAthena/pull/58)

* Fix to make it None when SQLAlchemy's URI username and password are empty strings.
If you do not specify `aws_access_key_id` and `aws_secret_access_key` in SQLAlchemy using the instance profile or boto3 configuration file, the connection string is as follows:

awsathena+rest://:athena.{region_name}.amazonaws.com:443/{schema_name}?s3_staging_dir={s3_staging_dir}&...

Thanks leahein. (laughingman7743/PyAthena/issues/54)
* Fix double escaping of percent character in SQLAlchemy.
Thanks mister-average. (laughingman7743/PyAthena/issues/56)

* Fix condition of has_table method in SQLAlchemy.

* Fix Pandas to lazy import.

* Implement PandasCursor.
Thanks jkleint. (laughingman7743/PyAthena/issues/46)
* Support TIME and JSON type.
* Fix execute method of cursor object to return self (cursor object).