Update function working.

I separated the business logic into a new file, `game-of-life.clj` so as not to litter the code with tests of our functions.

Our previous update function wasn't working because we weren't feeding every row/column combination into our new-val function. To remedy this, I built the new-row function which will return an updated row, given a matrix and a row number.

I also cleaned up sum-excluding-middle a bit :bowtie: