Adding puzzler 31: 'Double Take'

New version of https://github.com/scalapuzzlers/scalapuzzlers.github.com/pull/62 that is intended for [the 2.9 Archive](http://scalapuzzlers.com/archive.html). Since 2.9.x still seems be relatively widely used, I think puzzlers for 2.9 still have some relevance.

The changes for 2.10 are the basis for https://github.com/scalapuzzlers/scalapuzzlers.github.com/pull/65