Removed obsolete db::doQuery() calls with db::doPreparedQuery() calls
Introduced interface DatabaseInterface; added execute() method to vxPDO
Previous preparedExecute() calls replaced by "new" execute() method
calls; minor bugfixes and code cleanups
Replaced Mysqldbi references and type hints with DatabaseInterface and
vxPDO references
Composer now enforces PDO
Re-replaced DatabaseInterface type hints with vxPDO hints