Added toContainNumberOfElements-matcher

I found myself writing many specs checking that an element contained the correct number of buttons, or options in a dropdown-list.

So instead of writing 
```
expect($aDiv.find('button').length).toBe(2)
```
I can now write
```
expect($aDiv).toContainNumberOfElements(2, button)
```

The name of the function is somewhat long, but I find it's easier to see what I'm trying to test at a glance