Remove support for zero-argument cases in product, sum macros

Based on feedback from @cibernox in #81, the zero-argument cases for `sum` and `product` are too clever. When you do the following, errors should be thrown in development mode

```js
Ember.Object.extend({
  prop: not()
});
```

```js
Ember.Object.extend({
  prop: product()
});
```

The error thrown is `Error: At least one argument is required`.