Sum (Addition)

Similar to #48, in that it supports composable macros

```js
MyType = Ember.Object.extend({
  d: sum('a', 'b'),
  e: sum('a', 'b', 'c'),
  f: sum('a', 'b', 'c', 2),
  g: sum('a'),
  h: sum(2),
  i: sum(),
  j: [1, 2, 3, 4],
  k: sum(Ember.computed.max('j'), 5),
  l: sum(sum('a', 'b'), 5),
  m: sum('j'),
  n: sum('a', 'b', 'j')
});
```