Several slightly advance usage of Ruby

Demonstrating these Ruby concepts:
* range
* Enumerable#inject
* "Opening" existing class
* Hiding test suite in conditional

And, replacing TABs with double-spaces.
I like not needing to `reverse` or shovel in the extra 0 though!

Perhaps renaming `memo` to `result` would make using [`inject`](http://www.ruby-doc.org/core-2.1.0/Enumerable.html#method-i-reduce) a touch clearer?