[FEATURE] 3 new methods to Ember.Enumerable (2)

I couldn't modify this [pull-request](https://github.com/emberjs/ember.js/pull/3967), so had to create new one.
Copy description:

**compactBy**
Returns a copy of the array without elements with `key` equal to `null` and `undefined`.
```javascript
var arr = [Ember.Object.create({a: null}), {a: 1}, {a: false}, {a: ''}, {a: undefined}, {a: 0}, {a: null}];
arr.compactBy("a");  // [{a: 1}, {a: false}, {a: ''}, {a: 0}]
```
**maxBy**
Returns an object with maximum `key` value or `null` if all elements don't have `key`
```javascript
var arr = [{a: 2}, {a: 1}, {a: 3}, {a: 4}];
arr.maxBy("a");  // {a: 4}
```
**minBy**
Returns an object with minimum `key` value or `null` if all elements don't have `key`
 ```javascript
var arr = [{a: 2}, {a: 1}, {a: 3}, {a: 4}];
arr.minBy("a");  // {a: 1}
```
I would personally prefer a non-single digit variable name here.  Something like `object` or `item` seems good...