Rename Formula to FormulaType.

This is to make way for a "Formula" class that will define the data
structure for a series of subclasses. Since "Formula" is one of the
major nouns of your project, it probably deserves its own data type. So
that's what I'm going to look at next.
Create a basic Formula class, and a stubbed-out child Formula.
Add hook to intake's didSet property method to recalculate.

This adds a hook where you can define all your calculation logic in your
subclasses by overriding recalculate(), which I'll do next.

This method returns nothing, since it changes the state of the instance
of the formula. If you want to get the formula's new values, ask for
each of them individually after calling recalculate().
Add Intake enum, switch calculation to use value derived from switch.

The Intake enum represents the two different states that a user's input
can be in. This is a bit more "advanced Swift," but since it can either
be a "direct" milliliter value, or a calorie-based value, those are the
two different conditions. If the user inputs a calorie-based value, we
get the milliliter value by dividing the user's input value by the
number of calories per milliliter, as defined by the Formula subclass
(which you'll see in Formula.milliliters()). We then use this
"milliliters" value in the calculations, which I'll be changing up here
shortly.
Move from stored/defined properties to calculated ones.
Add additional formulas derived from original Calculator class.
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.
Re-wire ViewController to pull values on EditingDidChange event.
Merge pull request #2 from kreeger/formula-class-hierarchy

Formula class hierarchy