Adding hasClass/dosntHaveClass asserion

This PR is related to following issue https://github.com/dalekjs/dalek/issues/128
In PR was added two method that allow to test if element contains/don't contains class
 
```javascript
test
.open('http://dalekjs.com/guineapig/')
    .assert.hasClass('button.submit-product', 'plain-btn', 'button has plain-btn class')
    .done();
```

```javascript
 test
   .open('http://dalekjs.com/guineapig/')
   .assert.doesntHaveClass('button.submit-product', 'btn', 'button doesn't have btn class')
   .done();
```