SQL capitalization and several bug fixes for SQLite, MySQL and PostgreSQL

This pull should resolve a large number of issues currently open:
https://github.com/franz-see/Robotframework-Database-Library/issues/33
https://github.com/franz-see/Robotframework-Database-Library/issues/26
https://github.com/franz-see/Robotframework-Database-Library/issues/24
https://github.com/franz-see/Robotframework-Database-Library/issues/22
https://github.com/franz-see/Robotframework-Database-Library/issues/19
https://github.com/franz-see/Robotframework-Database-Library/issues/18

as well as a lot of cleanup work.