Etlhelper

+ Rename `sql_server` to `mssql` (5)
+ Add Python 3.8 to Travis (20)
+ CI fixes (fix Docker version, pytest deprecation warning fixes)
+ DbParams raises `AttributeError` on bad key

+ Update CI to allow push-button releases

- More examples in README
- Added logging functionality

+ Add SQLite helper
+ Improve definition of DbParams via environment variables

+ Add DbParams parameter validation
+ Make iter_chunks return chunks as generators
+ Add Oracle integration tests
+ Add FOSS4G links to README
+ Update CONTRIBUTING