Bug fixes & cleanup

[mc2contributor]

Fixes issues found when running the code in PHP 8 (only for scripts we've used):

• Replace deprecated mysql driver with mysqli
• Replace $array{$index} with $array[$index]
• Pass '' instead of null to functions that expect strings

Improves code by refactoring, especially reducing redundant wrapper XML.

Automatic debug logging when $log_level = QUICKBOOKS_LOG_DEVELOP.

The change in README.md should not be pulled, but I don't know how to exclude it.