Deprecate sumBy macro

Fixes #69 

Here's what the deprecation warning looks like on modern browsers. On older versions of IE, it'll fall back to `Ember.Logger.warn`
![screen shot 2014-10-14 at 1 30 29 am](https://cloud.githubusercontent.com/assets/558005/4626019/fc71f108-537c-11e4-84b8-a405bea7d816.png)