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)

There is already a `Ember.deprecate` function that does this kind of clever checkings, but the ember approach is that in the production builds is removed, so it only warns to you. 

I've just checked it, and seems that ember-cli does not remove it when building apps/addons. I'll ask this evening (London time) in the IRC if there is some kind of broccoli plugin that does that easily and we can use the built-in function. 

If not, we can use this. BTW, don't you think that this helper could go inside `utils.js`?