Do we want to put an example using this helper and what it'd do?

ie:

```HTML
<ul>
<li>Ember</li>
<li>is</li>
<li>great!</li>
</ul>
```
then 
```javascript
shouldHaveElementWithCount("ul li", 3); // will make the assertion that there are three, and the test will pass
```

Happy not to, just a possibility.