Ignore internal mysql databases when creating a backup

In my cases I usually don't want to backup the internal event + information_schema table of mysql, since backupping them raises the following error in:
``` console
# /usr/local/sbin/mysqlbackup.sh
-- Warning: Skipping the data of table mysql.event. Specify the --events option explicitly.
# /usr/local/sbin/mysqlbackup.sh
mysqldump: Couldn't execute 'SELECT /*!40001 SQL_NO_CACHE */ * FROM `INNODB_BUFFER_PAGE`': Access denied; you need (at least one of) the PROCESS privilege(s) for this operation (1227)
```

This small pull request, uses the INFORMATION_SCHEMA table and excludes "information_schema", "performance_schema", "test", "mysql" from the dumped databases.