Ruby Idioms: Simplify definitions
Ruby Idioms: Improve idiomatic and non-idiomatic examples

Partly based on feedback from JEG2.
Ruby Idioms: Move feedback slide to end

Also move "follow along" link to title page.
Ruby Idioms: Clean up idiomatic Ruby some
Ruby Idioms: Add memoization section

James Edward Gray II recommneded adding this to the presentation.
Thanks, James!