Difference (subtraction)

```js
  var MyType = Ember.Object.extend({
    c: EmberCPM.Macros.difference('a', 'b'), // a - b = 6 - 2 = 4
    d: EmberCPM.Macros.difference('a', 3), // a - 3 = 6 - 3 = 3
    e: EmberCPM.Macros.difference(3, 'a'), // 3 - a = 3 - 6 = -3
    f: EmberCPM.Macros.difference(), // 0
    g: EmberCPM.Macros.difference('a'), // a = 6 = 6
    h: EmberCPM.Macros.difference(Ember.computed.alias('a'), 2), // a - 2 = 6 - 2 = 4
    i: EmberCPM.Macros.difference(EmberCPM.Macros.sum('a', 'b'), 4) // (a+b) - 4 = 8 - 4 = 4
  });

  var myObj = MyType.create({
    a: 6,
    b: 2
  });
```

Supersedes #22