Use one global random object instead of one per Thing

It was originally the intention to have each Thing have its own Random object, so state could be saved between them, but in hindsight that's kind of pointless, I think.

Also, move the Fisher-Yates shuffle to the Random object, which is way more where it belongs.
Add three different types of planet + make life a bit more common

It turns out that the Goldilocks zone was hardly ever hit anyway, so there's almost no point putting the coinToss check in there as well.
Merge branch 'master' of github.com:ogham/totality