Fix:create custom assertions via callback

Implemention of the following feature https://github.com/dalekjs/dalek/issues/123 - Custom assertions via callback

Examples of using 
```javascript
test
    .open('http://dalekjs.com/index.html')
    .assert.evaluate(function () {
      return document.getElementsByClassName('grid').length;
    }).is(2, 'Count of grid on page is equal 2');
```