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)

Apparently they use `broccoli-defeatureify` to remove debug/warn statements. 

I think there is a problem if we use `Ember.deprecate`. 

Since ember-cli does not run defeatureify, probably ember-cli apps using ember-cpm will contains warning even built with --environment=production.
I have to check if plugins have the ability of add filters on its own trees before being included in the app, and if so, change the plugin to defeatureify itself in production environment.

Anyway, the important thing is deprecate this. Let's fix this later.