Array.prototype.$groupBy()

It's often helpful to be able to take an array and turn it into an
object whose keys are some computed value and whose values are the
elements of the array that correspond to that value. Such that:

["cat", "dog", "turtle"].$groupBy(function(animal) {
  return animal.length;
})

returns:

{ 3: ["cat", "dog"], 6: ["turtle"] }

Closes #22, https://github.com/canaryup/sprinkles/issues/22.