Add option to enable/disable logging of invalid nesting.

Some Magento installations tend to have invalid nestings on every single page. (I have to verify if this also happens for vanilla installations). However, this can bloat system.log rapidly when used on production systems.

I added the option to enable/disable the logging of invalid nesting. By default, the logging is enabled so default behaviour doesn't change.

I didn't bump the version number in case you want to do that.