Wire up new Calculator class method to view controller.

Since our Calculator just serves to talk between our View Controller and
our various Formula subclasses, it doesn't need to keep track of any
internal state anymore, and thus we can make our interation with it into
a class method, instead of an instance method. Again, it's partially
semantics here, but it's also good to reduce app state wherever possible
(it's a widely-held practice in iOS and other client-type apps that
application state can introduce nasty bugs). Following in this goal, I'm
gonna tweak a couple of things in my next commit to make it to where you
don't have to pick a calculate button.