Right Option key is almost useless for me and the built-in keyboard lacks a right Ctrl key.  I find Karabiner. It works perfectly!

[2014.9.25 update] I find Karabiner could alter behavior of  USB mouse. As we know, ‘Scroll direction: natual’  is really nature for MacBook’s trackpad, but not that nature for mouse wheel.  Karabiner could retain normal wheel behavior while keeping trackpad ‘natural’

My setting is attached