Implement universe.numberOfAliveNeighborsOf(), refactor Evolution test descriptions, add test for dead cell becoming alive.

The way I've implemented this before is with strings, so "1,2" instead of [1, 2].

I didn't know this about JavaScript before, that [0, 1] != [0, 1]. I might take a peek at how others have handled this before. We can share what we learn about it afterward.