option for simply just count the db->log entries.

This addition enables you to just count the logged entries in the db profiler, so `$db->log( true );` will return the number of executed queries, instead of all the log as string.

requested at the google group https://groups.google.com/d/msg/f3-framework/REUl8Uq3l6o/OIAmYW7zhDYJ