In case you ever run into an error Unknown table engine 'InnoDB' after tuning / tweaking your MySQL configuration, here's how to fix it easily:

(Please note, that we're going to clear all of your ib_logfile. Yes — it's usually safe to delete them as soon as the database isn't running anymore. They are getting recreated with the 3rd step)

service mysql stop
rm /var/lib/mysql/ib_logfile*
service mysql start

Et voilà! :)